His father was a taxi driver who subjected Martin's mother to domestic abuse and had spent periods of time in prison.

[4] Martin has been a vegan since 2014, initially for health reasons as he sought to manage issues with ulcerative colitis.

[10] He made his debut in a 2–1 home victory over Cambridge United on 7 August 2004, then went on to make ten more appearances during the 2004–05 season.

[16][17] Following Mark Cooper's appointment as a replacement for Darren Ferguson in November 2009, Martin joined Norwich City on loan.

On 7 May 2011, he was the runner up for the Norwich City Player of the Year award to winner Grant Holt, in a season where he played every minute of each game.

[24] After an impressive first season in the Premier League with Norwich, filling in well at centre back during some games, Martin signed a new three-year deal in June 2012.

[38] Following his loan with Rangers, Martin left Norwich City on 31 August 2018 after his contract was terminated by mutual consent.

[40] He left by mutual consent in January 2019, for family reasons, having made twelve appearances for the club.

[43] Following the departure of manager Paul Tisdale on 2 November 2019, Martin was appointed as his successor in his first managerial position the following day and later announced his retirement as a player to concentrate on the role.

He remarked before a game against Croatia in June 2013 that he was not recognised by taxi drivers when he met up with the Scotland squad.

[50] Martin, alongside assistant Luke Williams, installed a possession-based style of play at the club which received widespread plaudits.

[52] At the culmination of the 2020–21 season, only Manchester City and Barcelona had a higher average possession percentage in Europe than Martin's MK Dons.

[54] On 31 July 2021, MK Dons confirmed that Championship club Swansea City had requested to speak to Martin about him becoming their new manager.

[60] In October, Martin led Swansea to a 3–0 win against rivals Cardiff City in the South Wales derby.

[62] Martin said, "I have not felt as emotional as this after a game... to watch the backroom staff and the players enjoy that with the supporters was the best moment I've had as a manager I'd say.

[66][67] His first professional game in charge was a 2–1 victory against Sheffield Wednesday, ending Southampton's nine-match winless run in their last nine opening day fixtures.

[71][72] Martin led Southampton to play-off victory on 26 May 2024, beating Leeds United 1–0, securing promotion to the Premier League at the first time of asking.

My style is probably shaped by that and the teams I've enjoyed watching play football like Barcelona, Manchester City and Spain.

[80][81] His managerial style has been influenced by such managers as Chris Hughton, Daniel Farke, and Pep Guardiola.

[52][82] If his teams suffer a bad result, Martin prefers to improve his game plan, rather than changing the style of play.

[83] After his MK Dons side scored a record-breaking 56-pass goal, Martin said, "People still react to that and say it's boring, but everyone has a different opinion... the more we have the ball, the more we can control the game and take the fight out of opposition teams.