[5] He finally made his first-team debut on 31 October 2007, in Arsenal's 3–0 League Cup win over Sheffield United, when he was brought on to replace Theo Walcott in the 83rd minute.

[18] After impressing against Spurs, Lansbury went on to be named in the Arsenal squad for their UEFA Champions League game against Shakhtar Donetsk but did not make an appearance off the bench.

After a successful loan spell at Norwich City for much of the 2010–11 season, making 23 appearances and being involved in nine goals, he returned to his parent club.

[20] He was given the number 12 shirt and made his debut in the East Anglian derby victory against Ipswich Town on 28 November, setting up a goal for Grant Holt in the process.

[26] Lansbury missed out on the 2010–11 Football League Championship Young Player of the Year award after fierce competition from Connor Wickham who at the time was playing for Ipswich Town.

[27] He made his debut on 10 September 2011 in a 4–3 home win against Portsmouth scoring West Ham's second goal, eventually finishing with the Man of the Match accolade.

West Ham went on to win the game 4–1, and fans chanted "England's Number 1" at the final whistle in tribute to Lansbury's second-half goalkeeping debut, keeping a clean-sheet in the process.

On 28 August 2012, Lansbury joined Football League Championship side Nottingham Forest from Arsenal on a four-year deal[33] for a transfer fee believed to be in excess of £1 million.

[37] Having established himself as a regular starter for Forest, Lansbury missed the final months of the 2013–14 season through injury and was strongly linked in the summer with a move to newly promoted Premier League side Burnley.

[39] He made his return from injury on 26 August 2014, scoring and winning the fan's man of the match award in a 2–0 League Cup second round victory over Huddersfield Town.

Despite the continued media speculation and negotiations between Forest and Burnley, Lansbury's manager Dougie Freedman praised his new captain's attitude and commented that his performance against Rotherham was a "delight to watch".

[43] Less than a week after the Rotherham game, a statement was released by Forest on 20 August 2015 confirming that Lansbury had declared his intention to stay at the club.

Lansbury continued to be a first-team regular during the 2016–17 season under the new management of Philippe Montanier, and scored his first career hat-trick on 25 November 2016 to inspire Forest to a 5–2 defeat of Barnsley at Oakwell.

His outstanding performance in Forest's win over Barnsley was cited as a factor in him being bestowed with the honour by Sky Sports pundit Don Goodman.

[48] In January 2017, and with his contract situation unresolved, Lansbury was the subject of bids from Derby County and Aston Villa, the latter of which was reported to be worth £3 million.

[49] Amid the speculation, Lansbury missed Forest's FA Cup defeat at Wigan Athletic on 7 January because his "head was not right to play".

[50] On 20 January, after reported interest from Watford, Celtic and Norwich City, Lansbury completed a transfer to Aston Villa to bring an end to his four-and-a-half-year stay at Forest.

[53] However, from the beginning of the 2018-19 EFL Championship campaign onwards, Lansbury found it difficult to find regular game time, playing mostly in cup competitions.

At the start of the 2020–21 Premier League season, Lansbury found himself behind John McGinn, Conor Hourihane and Jacob Ramsey in the pecking order.

[63] Lansbury also held the captain's armband at the U-19 level where he took England to the 2009 UEFA U-19 European Championship final against Ukraine where they emerged as runners up.