Carew began his career with his local club Lørenskog, a minor club in the Akershus region. He was already considered a highly talented youngster and had gained some media attention before he was acquired by Vålerenga in 1997, and in the same year Carew was a part of the team that gained promotion to Tippeligaen and won the Norwegian Football Cup. During his two-year period at the club he played 58 matches and scored 30 goals, while his profile rose even more due to his combination of strength and goal-scoring ability.

John Carew played 91 times for Norway, scoring 24 goals, and was part of their Euro 2000 squad. He made his full international debut on 18 November 1998, and was the first black player to represent Norway.

After a string of impressive displays in the Champions League, Carew moved to Spanish club Valencia in an €8.5 million transfer, where he managed to win the La Liga title twice. Carew was part of the Valencia side that lost on penalties in the 2001 UEFA Champions League Final, though he scored his own penalty attempt. His goals in the campaign were crucial, including a 75th-minute header in a 1–0 home win against Arsenal in the second leg of the quarter-final, which saw Valencia advance to the semi-final. In the 2002–03 Champions League campaign, Carew was once again responsible for Arsenal’s exit from the competition. With Valencia needing a win in their final game of the second group phase at home against Arsenal to progress to the quarter finals, Carew scored twice in a 2–1 victory. He then spent the 2003–04 season on loan at Italian side Roma where he scored 7 goals in 26 games. Carew then moved to Turkey in 2004 to play with Beşiktaş. After the 2004–05 season, he was snapped up by Lyon for €7.6 million. While Carew was playing in Europe he was constantly linked with a move to the English Premiership and several failed attempts were made to sign him. He failed a medical at Fulham in 2002 and West Bromwich Albion had a bid rejected by Valencia.

John Carew has a son born in 2004 who lives with his mother in Norway.

Carew is noted for being a practicing Christian and often makes donations to charity. He also contributes to organizations like Soccer Against Crime, MOT and Ungdom mot Vold (Youth Against Violence) and was awarded the Kniksen award as Kniksen of the year in both 2005 and 2007. Since joining Aston Villa, Carew is noted for visiting children in hospitals around Birmingham and supporting Villa’s initiative to donate from the payroll towards hospital running costs.

On 22 January 2007, Carew signed for Aston Villa in an exchange deal with Lyon for Milan Baroš. Carew penned a three-and-a-half-year deal at the Birmingham-based club. Carew went on to receive the No. 10 jersey left vacant by Baroš’ departure.

Carew scored two goals against Reading on 12 January and was unlucky not to be awarded Man of the Match which went to Martin Laursen. He scored his first hat-trick for seven years and his first ever for Aston Villa against Newcastle United on 9 February in a 4–1 win. Gareth Barry allowed him to do so by kindly relinquishing his usual penalty taking duties so Carew could score his third. On 12 April 2008, Carew scored for Aston Villa against Derby County at Pride Park in the Premier League, in the 26th minute and Villa went on to win the game 6–0. On 20 April 2008, Carew scored twice against Birmingham City in the Birmingham derby at Villa Park, which Villa went on to win 5–1. He continued his scoring run with a header the following week, in a crunch game at Everton’s Goodison Park, which finished 2–2. It would be his 13th and final goal of the season, crowning him as Villa’s top scorer for the 2007–08 season.

Carew scored his first goal of the 2008–09 campaign by scoring the first goal in Aston Villa’s 2–2 draw over Odense in the Intertoto Cup. On 14 August 2008, Carew signed a 12-month extension to his contract, thus taking him through to 2011 with his current deal.

Carew scored the opening goal in Villa’s 4–2 win over Manchester City at Villa Park. He then scored his second league goal against Stoke City as they went on to lose the match 3–2 when Mamady Sidibe scored a late winner. He quickly added his 3rd of the season in the 2–1 defeat of local rivals West Bromwich Albion; and with Gabriel Agbonlahor on the scoresheet again the pair began to form a formidable partnership. They both scored again and assisted each other’s goals in a 4–0 victory in the Premier League at Wigan Athletic on 26 October 2008. Carew scored the winning goal in the UEFA Cup for Villa in a Group F match away to Slavia Prague on 6 November 2008, Villa midfielder Steve Sidwell struck the ball towards goal but it hit Carew and went in, therefore the goal was credited to Carew; it turned out to be the winning goal and continued Villa’s 100% record in the competition’s group stages and the 1–0 victory saw Villa go to the top of the group. Carew made the headlines in late October due to his personal behaviour. He was fined two weeks wages by Martin O’Neill for being in a pub near a Birmingham lap dancing club the night before Villa’s UEFA cup group stage match with Ajax.

Carew’s sister, Elisabeth Carew, is an R&B singer. She released her debut album, Destructive, in 2008. She is competing in Melodi Grand Prix 2014 with her song “Sole Survivor”.

Soon after the controversy, Carew sustained a back injury that kept him out of the side for several months. During this time, Villa boss Martin O’Neill signed England international striker Emile Heskey to fill the gap left by Carew’s absence. Heskey appeared to have taken Carew’s place in the starting line-up for a number of weeks. However, his own injury woes and Carew’s good form on return meant the Norwegian regained his place in the side. Carew played his first game after his injury on 31 January 2009 in a goalless draw with Wigan Athletic. In the last 32 of the UEFA Cup, Carew earned Villa a first leg draw with CSKA Moscow, after going 1–0 down to Vágner Love’s goal. On 1 March 2009, Carew came off the bench to score a lob-shot volley in the 2–2 draw against Stoke City in the Premier League, which was later voted the team’s goal of the season. He scored an equalising goal in the away fixture against Manchester United at Old Trafford before also netting the first goal in the home tie against Everton as Villa fought back from 2–0 and 3–1 down to draw 3–3. Further goals against Hull City and Middlesbrough took his league total to an impressive 11 goals from just 25 appearances.

Due to the persistence of fellow strikers Heskey and Agbonlahor, Carew initially struggled to hold down a regular spot in the starting eleven at the beginning of the 2009–10 campaign. Nevertheless, while being used as a substitute Carew still managed to score several important goals for Aston Villa. On 7 March 2010, manager Martin O’Neill chose to include Carew in the starting eleven in an FA Cup game against Reading. He took the opportunity characteristically and scored a hat-trick as Aston Villa came from two goals behind to defeat Reading 4–2 in the FA Cup quarter final. This made Carew the competition’s top scoring striker, raising questions as to why the Norwegian was rarely included in Villa’s starting eleven. Carew began to feature in the Villa team once more as the season progressed; goals against Wolverhampton Wanderers, Sunderland and Chelsea helped the Norwegian go neck and neck with Gabriel Agbonlahor as the club’s joint top scorers for 2009–10.

In December 2010 Carew was a victim of fraud after he paid £100,000 for a Porsche Cayenne Gemballa which never arrived. In 2012 Carew was declared bankrupt following a petition by HM Revenue and Customs. The Bankruptcy Order was annulled on 4 July 2012.

On 27 May 2011, Aston Villa announced that Carew was one of a number of players released by the club after their contracts expired.

Carew joined Stoke City on loan until the end of the 2010–11 season on 21 January 2011. Following his move Carew stated that he was looking forward to a new challenge and also revealed that he rejected offers from other clubs. His arrival has been welcomed by the Stoke players including Andy Wilkinson.

Carew made his debut for City the following day at Fulham where he came on as a substitute. He scored his first goal for Stoke in a 3–2 win against Sunderland on 5 February 2011. He followed this up by scoring in the FA Cup against Brighton & Hove Albion. However a back injury slowed his progress in a Stoke shirt. After his release from Villa, Tony Pulis did not offer Carew a contract with Stoke.

On 6 August 2011 Football League Championship side West Ham United announced the signing of Carew as a free agent. He scored his first goal for West Ham in a 2–2 draw with Crystal Palace on 1 October 2011. On 23 May 2012, Carew was released with five other players including Julien Faubert, Frank Nouble, Papa Bouba Diop, Abdoulaye Faye and Olly Lee. After the 2011–12 season, Carew did not get a new contract with West Ham.

In August 2012, Carew was in talks with his old club Vålerenga, but Vålerenga did not accept his demand of a wage of 600,000 kr per match. In connection with Carew’s possible transfer to Vålerenga, Kjetil Siem told Dagbladet that he had signed an unofficial contract with Carew in 2005, when Siem was Director of Football in Vålerenga, that Carew would return to Vålerenga in 2012. He went on trial with Serie A team Internazionale in February 2013, but was not offered a contract as Inter were concerned over his fitness. In October 2013 Carew stated that he had retired.

In 2014, Carew starred in his first feature film, a Canadian horror called Dead of Winter. His second film, a Norwegian-made thriller called Høvdinger, was released on 17 July 2015.

In 2018, Carew starred in the hugely successful Norwegian TV-series, Heimebane, playing as Michael Ellingsen, an aging star footballer for a local Norwegian club.

In 2019, Carew played the role of Jungle Warrior in the Disney’s dark fantasy adventure film, Maleficent: Mistress of Evil alongside Angelina Jolie, Elle Fanning and Chiwetel Ejiofor.