Lallana was a member of the AFC Bournemouth Centre of Excellence, where he was spotted by a talent scout from Southampton. He joined the Premier League club’s Saints Academy as a 12-year-old in September 2000. Southampton paid £3,000 compensation to Bournemouth with further payments of £5,000 and £10,000 when he signed scholarship and professional contracts respectively.

Lallana joined the first team squad in July 2006, making his debut on 23 August 2006, in a 5–2 victory over Yeovil Town, in the League Cup. On 31 October 2006 he signed a new contract to keep him at Southampton until December 2009. On 9 October 2007 he joined Bournemouth on loan for one month. On 28 April 2008, Lallana scored his first professional goal for Southampton against West Bromwich Albion in a crucial Championship match, with a well-placed finish into the bottom left corner.

He became a regular in the first team at the start of the 2008–09 season. On 29 August 2008, he signed a new three-year deal with the Saints.

On 7 January 2011, Lallana signed a new contract with Southampton which would expire in the summer of 2015. He was named in the League One PFA Team of the Year for the 2010–11 season after scoring 11 goals that season.

Lallana played for England Under-18s, scoring on his debut against Slovenia, 18 months after undergoing an operation to correct an irregular heartbeat. He also played once each for England Under-19s and England Under-21s. On 10 September 2012, Lallana was called up to the senior England squad for the first time for the 2014 World Cup qualifying match against Ukraine although he was not selected for the match itself.

He made his first Premier League appearance on the opening day of the 2012–13 season, setting up a goal and wearing the captain’s armband in a 3–2 defeat against 2011–12 champions Manchester City. He scored his first Premier League goal in a 4–1 defeat away to West Ham United. His second goal of the season came on 25 November, as he opened the scoring in a 2–0 home win against Newcastle United. The 50th goal of his career came in a 2–0 victory at Reading. On 11 April 2013, Lallana signed a new five-year contract with Southampton.

In Southampton’s 4–1 Premier League victory over Hull City on 9 November 2013, Lallana started with the ball 30 yards from goal and dribbled his way past five players into the left-hand side of the 18-yard box, before slotting it past Hull goalkeeper Steve Harper. On 18 April 2014, he was named as one of the six players on the shortlist for the PFA Players’ Player of the Year award. On 27 April 2014, he was named alongside teammate Luke Shaw in the PFA Team of the Year. On 6 May, he was crowned as both the Southampton Players’ Player and Fans’ Player of the year.

On 7 November 2013, along with Southampton teammates Rickie Lambert and fellow uncapped Saint Jay Rodriguez, he was called up to the England team. Lallana won his first cap for England on 15 November by starting a friendly match against Chile at Wembley Stadium. Although England lost 2–0, Lallana’s performance received generally favourable reviews. He also started in England’s defeat against Germany four days later. He gained his third cap in a 1–0 friendly win against Denmark on 5 March 2014, coming on as a substitute for Jack Wilshere and crossing for Daniel Sturridge to head in the only goal of the match.

Lallana married his fiancée Emily Jubb on 24 December 2013. The couple, who had been dating for seven years, were due to marry on 14 June 2014, which coincided with England’s opening World Cup match with Italy. They wed at Poole Register Office in Dorset. Together, they have two sons, Arthur Michael, born on 26 September 2012 and Albie George born on 3 June 2015.

On 12 May 2014, Lallana was named in the 23-man England squad for the 2014 FIFA World Cup. He made his competitive debut on 14 June in the first group match against Italy, coming on for the last 10 minutes in place of Sturridge who had scored England’s goal in the 1–2 defeat.

In 2014, Lallana was signed as a French Connection model, to promote their Autumn/Winter collection. He revealed his fashion photo shoot at an interview with Esquire magazine.

On 25 July, Liverpool confirmed that Lallana had injured his knee while training with the club in Boston, Massachusetts during pre-season and would be out for six weeks. He therefore missed the start of the league season, which began against former club Southampton. He made his debut in Liverpool’s fourth match on 13 September, starting and being substituted for Raheem Sterling after 61 minutes in a 1–0 home defeat to Aston Villa. On 4 October he scored his first goal for the club, opening a 2–1 home win against West Bromwich Albion seconds before half time. Lallana scored his second goal for Liverpool on 2 December during Liverpool’s 3–1 win at Leicester City. On 29 December, he continued his fine form and scored two goals in a 4–1 win against Swansea City at Anfield. On 14 February, Lallana scored the winning goal in a FA Cup Fifth Round match against Crystal Palace. On 19 April 2015, Lallana was selected in the Football Manager Team of the Decade at the Football League Awards.

On 17 September 2015, Lallana scored the opening goal in the new Europa League season opening group stage away match against Bordeaux at the Stade Chaban-Delmas, a match Liverpool went on to draw 1–1. On 1 October, Lallana scored in a second match in a row against Sion in the Europa League but the match ended in a 1–1 draw. On 23 January 2016, he came on as a second-half substitute and scored an injury-time winner in Liverpool’s 5–4 away win at Norwich City. On 28 February 2016, Lallana came on as a second-half substitute and missed in the penalty shoot-out against Manchester City, resulting in Liverpool’s loss in the 2016 League Cup Final. On 3 March 2016, he scored and assisted in Liverpool’s 3–0 home win against Manchester City. On 23 April, he scored Liverpool’s second goal against Newcastle United, which took the Reds to a 2–0 half-time lead, but the match ended with a 2–2 draw. On 5 May, he scored the third goal in Liverpool’s second-leg win over Spanish side Villarreal in the Europa League semi-finals, with the Reds advancing to the final with a 3–1 aggregate win.

On 14 August 2016, during the opening match against Arsenal, Lallana scored Liverpool’s second goal, which took the Reds to a 2–1 lead, with the match eventually ending in a 4–3 win for Liverpool. On 10 September, Lallana scored in Liverpool’s 4–1 win over the previous season’s champions Leicester City. Two weeks later, on 24 September, he scored in Liverpool’s 5–1 win over Hull City. On 11 December, he scored Liverpool’s opening goal in the mach against West Ham United, which eventually ended in a 2–2 draw. Three days later, he scored twice in a 3–0 defeat of Middlesbrough, and on 27 December, he scored in Liverpool’s 4–1 win over Stoke City. On 22 February 2017, Lallana signed a new long-term contract with Liverpool. On 21 May 2017, on the final day of the season, Lallana scored Liverpool’s third goal in their 3–0 win over Middlesbrough, which secured a top-four finish for the Reds and qualification to next year’s UEFA Champions League.

Lallana scored his first goal for England on 4 September 2016 in a 2018 FIFA World Cup qualification match away to Slovakia at Štadión Antona Malatinského. It came during stoppage time and proved to be England’s winning goal of the match. He scored his second England goal on 11 November 2016 in a 2018 FIFA World Cup qualification match against Scotland. The score ended 3–0 to England, with Gary Cahill and Liverpool teammate Daniel Sturridge scoring the other goals.

In August 2017, Lallana suffered a long-term injury during the Audi Cup final against Atlético Madrid, which would keep him out until mid-November. His first appearance of the season was in Liverpool’s 1–1 draw with Chelsea, coming on as an 89th minute substitute for Philippe Coutinho. On 31 March, during a match against Crystal Palace, Lallana came on as a substitute in the 65th minute, but only several minutes after playing, he suffered another injury, which kept him out for “a number of weeks”. He returned to the squad on 13 May, on the final day of the Premier League season, coming on as a substitute for Sadio Mané in the 74th minute, playing against Brighton & Hove Albion. Liverpool won the match 4–0, which was enough to secure a top-four finish and a place in next year’s UEFA Champions League group stages, as finishing in fourth place now meant that clubs in the Premier League will automatically qualify for the group stage, rather than qualifying initially for the play-off round.

On 30 January 2017 it was announced that Lallana had won the England Player of the Year Award for 2016 after his performance for the national team.

On 16 May 2018, he was one of five players named on standby for the 23-man England squad for the 2018 FIFA World Cup.

On 20 October 2019, Lallana came off the bench and equalized for Liverpool against Manchester United at Old Trafford in the 85th minute, his first goal in over two years, earning his side a 1–1 draw and helping them remain unbeaten in the league.

On 9 June 2020, Lallana signed a short-term contract extension with Liverpool, keeping him at the club until the end of the 2019–20 season which had been interrupted by the COVID-19 pandemic. On 25 June 2020, Liverpool clinched the 2019–20 Premier League title following Manchester City’s failure to beat Chelsea, marking their first league title in 30 years. Lallana left the club at the end of the season with his contract set to expire, ending his six-year stay at the club.

Lallana signed for Premier League club Brighton & Hove Albion on 27 July 2020 on a three-year contract on a free transfer. Lallana made his debut for The Seagulls on the opening day of the 2020–21 season starting in the 3–1 home loss against Chelsea. He later went off injured. He came on as a substitute in Brighton’s second league match, a 3–0 away win over Newcastle United.