Cook had his first taste of international cricket playing the U15 World Cup in 2000. Three years later Cook was called up to the England Under-19 team for their tour of South Africa. After it was decided Samit Patel had enough responsibility batting and bowling, Cook was appointed captain of the team the following year, 2004. His first call of duty was the U19 World Cup in the West Indies where he guided England through the group stages and on past the Super League stage, including an unbeaten century-each partnership with Patel against New Zealand, and continued a 100% win record with 87 over eventual winners Pakistan before falling short in the semi-final against the West Indies. He went on from there to captain them in a U19 Test win over Bangladesh before taking the One-Day series too. His career with the Under 19 team was cut short when Kevin Pietersen was called up to replace Andrew Flintoff in the main team, leaving a spot in England A tour of Sri Lanka for Cook.

Over the next four years, he hit 17 centuries and two double-hundreds to total 4,396 runs at an average of 87.90, captaining the cricket team in his final year under coach Jeremy ‘Boris’ Farrell, as well as being President of the Music Society. He also gained three A-Levels and nine GCSEs. In his final year at Bedford in 2003, he scored 1,287 runs for the school, including two unbeaten double-hundreds, averaging 160.87 to take the school record. After his international success, Cook returned for an Old Boys’ match at Bedford in 2008, playing for the Head Master’s Ultimate XI. Later on, Cook spent two years at Alleyn’s School as a PE master.

Having been a member of the Essex Academy since the age of 16, Cook was given his first-class debut for Essex against Nottinghamshire County Cricket Club on leaving school at the end of the 2003 season; put in to field he aided the first dismissal with a catch before scoring just 13 runs in his first innings but pushing Essex to a nine wicket victory with 69 not out in the second. Despite Essex’s relegation being assured, Cook still battled away with an opening stand of 122 opposite Will Jefferson in the second innings of his second match, against Warwickshire. In the six innings in which he participated he quickly racked up three half centuries at an average of 47.80. Having secured his place as opener in the county team, he made his maiden first-class century against Leicestershire in May 2004 with 126 opposite Jefferson’s 128 for a 265 opening stand, Essex’s third highest but the team failed to secure promotion.

His exploits in his single season gained enough attention for him to be brought in as opening batsman for the MCC in 2005 season’s opener against County Champions Warwickshire. With a century in the first innings and 97 in the second, Cook helped secure victory in a match destined for a draw. The season opener would not be Cook’s only highlight for the year: as the Ashes approached, Australia were touring the counties and in a two-day match (which did not count towards his first-class statistics) Cook scored a double century and went on to take the first wicket of Justin Langer with a catch in the drawn match days after being awarded PCA Young Cricketer of the Year. A further highlight of the season saw Cook help Essex to achieve one of Durham’s two defeats of the season, scoring 107 of Essex’s 245 before taking his maiden first-class wicket, followed by another two, with his off-spin. Cook played in every first-class match for Essex and also helped them clinch the Totesport League Title for the first time in 20 years. The end of the season saw Cook finish with an average of 48.03 in the County Championship, hitting four centuries.

After the 2005 domestic season he was part of the intake for the ECB National Academy, but this was interrupted as he flew to Pakistan to cover the squad for Michael Vaughan’s knee injury. After not being used in the first XI he travelled back with the Academy to the West Indies in the new year scoring 101 in a two-day match against Antigua before youth cricket came to an abrupt halt for Cook.

As well as the one day series against Pakistan, Cook was overlooked for the 2006 Champions Trophy squad. However, he was named for the 2006–07 Ashes series touring party and when Trescothick pulled out once again due to stress, Cook re-earned a spot as an opener for the foreseeable future. In the disastrous 5 Test whitewash, Cook failed to impress in the first two Tests, but during a two-day warm-up he retired with a century and afterwards remained adamant that he and England would make a turn around in the series. With England desperately chasing 577 runs, or a draw to not lose the Ashes, Cook stayed at the crease for over six hours, lasting almost all of the fourth day to earn his maiden Ashes century (119) before being caught behind off Glenn McGrath just shy of three overs before the close of play. This was his fourth Test century before turning 22, no England player had scored more than two by the same age. With only three other batsmen scoring double figures, England lost the Test and the Ashes and despite only having an average of 27.6 runs, Cook scored one of only three English centuries on the tour and also on 28 December 2006 in Melbourne, Cook scored his thousandth Test run, the second cricketer to do this in their maiden year, after Mark Taylor.

For his performances in 2006, he was named in the World Test XI by Cricinfo.

After his England call up, Cook’s appearances at Essex significantly reduced. In the County Championship he appeared in only three matches, making two hundreds and another two fifties in his five innings, topping the season averages with an inflated 141.3. Similarly in the Cheltenham & Gloucester Trophy he appeared only three times and again topped the batting averages, only being out once with a top score of 91 and average of 148. He scored less favourably in his one Twenty20 Cup appearance, scoring only nine runs. In the 2007 season, for Essex alone his first-class average was only 58.8 from three appearances, making two hundreds but countering this with one duck and he made a similar century/duck appearance in the (formerly C&G) Friends Provident Trophy during his 5 matches averaging 38.4. The back to back Totesport/Pro40 champions saw Cook make only one appearance in that competition, scoring 81 not out, before being relegated. They won Division Two and were promoted the following season, though Cook only provided 11 runs in one innings during the tournament. Despite his international commitments Cook managed to participate in at least one match of each tournament averaging 36.16 in three County Championship matches and 15 in his one Twenty20 match. Given time off the South Africa One-Day series, Cook fared better in the FP Trophy with 45.3 from 3 matches including contributing the second highest in the final at Lord’s to seal the trophy.

The 2009 English season featured an early Test series against the West Indies where, despite being in the Test team, Cook joined Essex in a tour match. He scored 76 not out in the second innings before the match was rained off, stopping Essex pressing for victory. Before the Test series he helped Essex force the follow on against Kent in the County Championship but faltered with his team, scoring only 4 in the second innings to lose by 192 runs. After the Test series, Cook was omitted from the one day side leaving him free to play for Essex. He joined them in their bid to defend their Friends Provident Trophy matching Varun Chopra’s 65 in a 124 run partnership against Lancashire to gain a place in the semi-final. He and his team could not recreate the same form for the semi-final, at the same location against the same team three days later, after being caught from a running catch by Sajid Mahmood, seeing the team lose by 67 runs. Having been knocked out of the FP Trophy, the team turned to the Twenty20 Cup. Making only his fourth appearance in the tournament since 2005, Cook hit 80 off 56 balls in a match against Kent that was rained off. While his England partner Bopara scored 45 against Sussex, Cook only scored one but made up for this with a 60 run partnership with Bopara days later to put Essex top of the table. Despite averaging 49 from these four games, Cook felt he was a long way from the international squad. In his last performance before joining the Ashes squad, Cook scored a 57 ball century, and batted throughout the whole innings as they beat Surrey, but without Cook and Bopara, the team were knocked out after two successive losses.

For his performances in 2011, he was named in the World Test XI by the ICC and Cricinfo.

In December 2011, Cook married Alice Hunt, whom he had known since childhood. The couple have two daughters. He was allowed to miss a tour game in Bangladesh to be present at the birth of the second child in 2016.

On 29 August 2012, Cook became England’s Test captain after previous Test captain Andrew Strauss retired from all forms of cricket.

Cook, who captained England in Bangladesh for two Tests in a previous series, took charge of England for the Test series against India in the winter of 2012. In each of the first three Tests – the first a defeat and the second a ten-wicket victory – he scored a century. This, together with a century in each of the aforementioned two Tests in Bangladesh, gave Cook five centuries in five Tests as captain, a record number. His scores on the tour also included 119 and 97 in the warm-up matches. The ten-wicket victory in the second Test was lauded by the media as one of England’s greatest victories, and Cook’s run of hundreds brought him level with the English Test record for most centuries. On 6 December 2012, Cook became England’s leading scorer of centuries after making a hundred against India in Kolkata.

On 7 December 2012 he captained England to its first Test series victory on Indian soil since 1984–85, as well as overtaking Mike Gatting as England’s highest ever run scorer in India For his performances in 2012, he was named both in the World Test XI and ODI XI by the ICC . He was also named in the World Test XI by the Cricinfo.

Cook led England in the 2013 Champions trophy, and they won their first game against Australia. It was also Cook’s first game as captain against Australia. Cook made 59 against Sri Lanka, but England lost the game meaning they were not assured of their place in the next round. Cook top scored with 64 as England narrowly beat New Zealand to book their place in the semi final. After beating South Africa in the semi finals, England played India in the final in what turned out to be a rain affected match. Despite restricting India to 129 in 20 overs, England lost the match by 5 runs, leaving them as runners up. Cook was named as part of the ‘Team of the Tournament’ by Cricinfo.

The 2013 home Ashes series marked Cook’s first Ashes series as captain. England went into the series as strong favourites, with Australia recently having parted company with their coach. England won the first Test, with Cook making 50 in England’s second innings to help them set Australia a competitive target to win the game. They were bowled out for 296, meaning England won the match by 14 runs. Although Cook only managed 20 runs in the next match, England cruised to victory, winning by 347 runs. The third match ended in a draw, with Cook scoring 62 in the first innings, but being dismissed for a duck in the second. The fourth Test saw England win the series after winning the match and taking a 3–0 series lead. The final match ended in a draw, with Cook failing to pass fifty in either of his innings. Despite this, Cook was generally praised for the way he had led the side. In the ODI series, Cook and Flower decided to rest several of England’s Test players to ensure they were fit for the series in Australia. Cook himself played no part in the series, being replaced in the side by Michael Carberry.

For his performances in 2013, he was named as captain of the World Test XI by the ICC.

On 19 December 2014, Cook was removed as one-day captain, and replaced by Eoin Morgan. This was primarily due to his poor form in ODI cricket.

Cook captained England in the 2015 Ashes series, and although England won the first game, Cook could only manage scores of 20 in the first innings and 12 in the second. England lost the second Test by 405 runs, but Cook was one of the few England batsman to show any resistance, scoring 96 in England’s first innings. With the series level Cook made 34 in England’s first innings of the third Test and England went on to win the game by eight wickets. England won the fourth Test by an innings and 78 runs, meaning Cook only needed to bat once, scoring 43 in England’s first innings. With the Ashes won, Cook was praised for his captaincy and he gave an emotional interview following the victory. England lost the final match of the series, but Cook was again one of few England batsmen to show any resistance with the bat as he scored 85 in their second innings. Despite the loss, England won the series 3–2 and Cook celebrated his second Ashes win as captain.

Cook scored his third double-century and 28th Test ton (263) against Pakistan in first Test at Abu Dhabi in October 2015 in 836 minutes: the longest innings by an Englishman and 3rd longest of all time, facing 528 balls. In doing so he also became the all-time highest non-Asian run scorer in Asia, overtaking Jacques Kallis’ 2,039 runs. Despite Cook’s brilliant innings, England were unable to force a victory, running out of time after bad light stopped play. In the second Test Cook scored 65 in the first innings as England posted 242. With England chasing down a big target to win in the second innings, Cook could only manage 10 and Pakistan won the match by 127 runs. England need to win the third Test to have any chance of drawing the series and despite taking a first innings lead which Cook contributed 49 to, Pakistan batted well in the second innings and England were dismissed for 256 in their second innings, meaning the lost the series 2–0, despite Cook scoring 63.

For his performances in 2015, he was named as captain of the World Test XI by the ICC. He was also named in the Test XI by ESPNCricinfo and Cricbuzz.

In the first Test against Sri Lanka, Cook could only make 16 but England went on to win the match by an innings and 88 runs. In the second Test, left-handed batsman created history on 30 May 2016 in the second Test against Sri Lanka to become the youngest batsman to score 10000 Test runs beating Sachin Tendulkar’s 11-year-old record. The 31-year-old broke the record on the fourth day of the second Test match against Sri Lanka at Chester Le Street. He became the 12th international cricketer to reach the landmark and the first Englishman. He scored 47 in England’s second innings to help England to a convincing nine wicket victory. In the first innings of the third Test, he scored 85 to help England to 416. He came in down the order for England’s second innings after being injured in the field and made an unbeaten 49. Rain on the final day meant the match finished in a draw, with England winning the series 2–0.

Cook broke India great Sunil Gavaskar’s record of 9,607 for the most runs as a Test opener while making 81 against Pakistan in the first Test at Lord’s. However, Cook could only make eight in the second innings and England lost the opening Test by 75 runs. On 22 July 2016, Alastair Cook equalled Don Bradman’s record of having hit 29 Test centuries when he scored 105 on the opening day of the 2nd Test against Pakistan at Old Trafford. He chose not to enforce the follow-on when England were far ahead in the second Test – but was vindicated by an England victory, with Cook scoring an unbeaten 76 in England’s second innings. In the third Test he made scores of 45 and 66 and England won by 141 runs to go 2–1 up in the series. However, England lost the final match, with Cook scoring 35 in the first innings before making seven in the second innings as England lost by 10 wickets and the series ended in a 2–2 tie.

For his performances in 2016, he was named as captain of the World Test XI by the ICC.

When Cook retired from international cricket in 2018, he stated that he would continue to play for Essex, signing a three-year contract with them. In the 2019 County Championship season Cook helped Essex to win the title In August 2020, during the Bob Willis Trophy, Cook scored his 24,000th run in first-class cricket.

Cook’s form slump that had continued through the 2017-18 Ashes, where he had not scored a half-century in 10 innings, finally came to an end with 104* from 166 balls by the end of day two of the Boxing Day Ashes Test in Melbourne. He went on to carry his bat with a double century and put England into a lead of over 100, finishing with 244 not out, the highest Test score by an opener who has carried the bat. In December 2018 this record was bettered by New Zealand’s Tom Latham, in his score of 264* vs Sri Lanka.

After a score of 70 of the first match of the home series against Pakistan, Cook had another slump in form, scoring a total of 155 more runs in 5 further matches Pakistan and India. On 3 September 2018, after the conclusion of the fourth Test, Cook announced that his twelve-year international career would end at the conclusion of the series against India. On the fourth day of the fifth Test (after resuming on an overnight score of 46*), Cook made a score of 147 and in doing so became the fifth batsman to score a century in his first and last Test match and the second batsman to score twin fifties in his first and last Test. When he reached 76 during the innings, he overtook Kumar Sangakkara to become the fifth highest run scorer and the highest scoring left-hander in Test history.