[6] Martin began his career with Conference South club Eastleigh as a 14-year-old,[7] having been tracked by the management team of Ian Baird and Mark Dennis.

[10] He signed a new two-year contract in September 2009 and featured alongside Tom Jordan another 14 times in the early stages of the 2009–10 campaign, scoring one goal in a 6–1 win over Bromley.

On 1 May, Martin made his debut first-team appearance for the Saints, starting alongside Fonte in the centre of defence against Gillingham in the season's penultimate game.

[18] Southampton manager Alan Pardew said he thought Martin was his team's best player in the game, adding that "he passed it well and looked comfortable in the shirt".

[18] Martin started just four league games in the 2010–11 campaign, as new manager Nigel Adkins preferred to partner Fonte and Jaïdi, with Seaborne as back-up.

[35] Two days after his release, he joined Championship club Birmingham City on a short-term deal until the end of the 2013–14 season, after manager Lee Clark had Dan Burn and Kyle Bartley recalled from their loan spells.

[36][37] He made his debut at St Andrew's on 1 February, coming off the bench after 14 minutes to replace the injured Tom Thorpe, in a 3–3 draw with Derby County.

[44] On 29 October 2014, Martin joined fellow League One side Coventry City on loan until 5 January, in a deal which saw Jordan Clarke move in the other direction.

[48][49] He was named as the Football League Unsung Hero of the Month for January after making several visits to schools in deprived areas around Coventry and donating £1,000 to a children's charity.

[50] In May 2016, manager Tony Mowbray opted to release Martin at the end of his contract due to his lack of aggression, citing it as his most difficult decision of the summer.

[51] On 25 June 2016, Martin joined League One club Oxford United on a two-year contract, citing manager Michael Appleton's style of play as a factor in his move to the Kassam Stadium.

[54] He struggled with minor injuries and was limited to 11 appearances in the first half of the 2016–17 campaign, before a ruptured Achilles tendon in December ruled him out for the remainder of the season.

[55][56] New manager Pep Clotet had stated that Martin would be one of four centre-backs in the 2017–18 season, along with Mike Williamson, Curtis Nelson and Charlie Raglan.

[73] He picked up a back injury early in the 2021–22 season, but scored his first goal for the "Valiants" in a 3–0 win over Colchester United at Vale Park on 23 October.

[78] In July 2022, Martin returned to Eastleigh after having left the club 13 years ago; manager Lee Bradbury noted that "he's a local lad coming home".

[79] Martin was announced as club captain ahead of the commencement of the 2022–23 season and went on to feature 36 times throughout the campaign, helping Eastleigh to a solid defensive record.