His career, during which he represented both Sporting and Benfica, was marred by injury and indiscipline problems. He also competed professionally in Spain, with Recreativo and Granada, and amassed Primeira Liga totals of 201 games and 18 goals over the course of 12 seasons.
A member of Portugal's team at the 2004 Olympics, Martins earned 17 senior caps for his nation.
Born in Oliveira do Hospital, Coimbra, Martins was a youth product of Sporting Clube de Portugal, joining its ranks at the age of 11. He made his professional debuts with modest S.C. Campomaiorense in the second division and, from January–June 2003, was again loaned, to Primeira Liga side Académica de Coimbra.
In the 2005–06 season, despite suffering from several physical problems as well as disciplinary issues, Martins was still able to score two goals in only 13 starts.
However, in February 2007, after several bouts of indiscipline at Sporting, coach Paulo Bento ostracized Martins for the final three months of the season, and although a player still under contract, moving abroad was the only option for the player and, in June, he transferred to La Liga team Recreativo de Huelva.
In November, Portuguese network RTP Internacional interviewed Martins about his situation at Sporting and the player responded: "My situation with coach Bento has been a very bad one. I was constantly pressured to perform and when I did eventually get the call-up to the national side, it got so far that it almost turned into a physical confrontation". Bento denied this despite media sources releasing information of the two that went back to the manager's playing career at Sporting.
Martins finished the season with seven goals in 35 competitive appearances. He was a fan favourite at the Nuevo Colombino and was dubbed their version of Steven Gerrard by media sources and fans, also being nicknamed Carlitos.
On 1 July 2008, Martins signed a five-year deal with S.L. Benfica, with the club paying €3 million for the transfer(40% to Recreativo, 40% to Sporting and 20% to the player). Regularly used during his debut campaign although never an undisputed starter, he netted his first goal for the team in a 2–0 win at Vitória de Guimarães for the Portuguese League Cup; also in that competition, he scored the decider in the penalty shootout final win over former side Sporting, on 21 March 2009.
Early into the following season, Martins lost some starting XI room, mainly due to recurrent injuries. However, in the second part of the campaign, he bounced back and figured prominently for Jorge Jesus' side. On 21 March 2010, exactly 365 days after the last final, he helped Benfica renew their League Cup supremacy, as he netted in a 3–0 win against FC Porto from a 30-meter free kick.
On 13 August 2011, Benfica loaned Martins to Granada CF – recently promoted to the Spanish top division – in a season-long move. He appeared regularly as a starter during his spell but, although the Andalusians attempted to renew his loan deal, the Portuguese rejected, only allowing the player to leave on a permanent basis.
After a successful 2012 preseason, where he scored several goals, Benfica renewed Martins' contract in August, until 2016. Appearing sporadically throughout the campaign, he did feature as a substitute in a match against G.D. Estoril-Praia on 6 May 2013: with the game tied 1–1 and Benfica looking to score, he was booked in the 70th minute and, eight minutes later, was sent off after another foul in an eventual home draw that eventually saw his team lose the title race to Porto. He was subsequently told to look for a new club after manager Jesus deemed him surplus to requirements but, after being unable to do so, he spent the entire 2013–14 with Benfica B in the second level, only appearing after the winter transfer window.
On 16 September 2014, Martins terminated his contract with Benfica. The following month he signed for one year with fellow league club C.F. Os Belenenses, making his official debut on 7 January 2015 by starting in a 1–7 away loss against S.C. Braga for the Portuguese Cup.
Martins scored a brace to help his team defeat IFK Göteborg 2–1 at the Estádio do Restelo on 30 July 2015, in the first leg of the third qualifying round of the UEFA Europa League.
After Portugal finished in fourth place at the 2006 FIFA World Cup Martins, a former under-21 international, was called to the senior squad for the first time for a friendly with Denmark and a UEFA Euro 2008 qualifier against Finland in September, making his debut in the former on the first day of the month.
Appearing rarely in the following four years, he received a surprise callup in September 2010 for the Euro 2012 qualifying campaign matches against Denmark and Iceland, as former Sporting coach Bento was in charge of the national team: he started in both games, consecutive 3–1 wins.
After featuring regularly in the qualifiers Martins missed the final stages in Poland and Ukraine however, after contracting an injury in late May 2012, already in training camp.
On 15 November 2011, during Portugal's Euro 2012 playoff home fixture against Bosnia and Herzegovina, it was announced that Martins' three-year-old son Gustavo suffered from a rare bone marrow condition, needing an urgent transplant. The player spent nearly two weeks in his country, returning to action for Granada (as a starter) in a home game against RCD Mallorca on the 20th, scoring the 1–1 equalizer and being booked shortly afterwards for a reckless challenge, with the game being eventually suspended minutes later after a 15-year-old vandal hit the assistant referee with an umbrella.
Martins' younger brother, João, was also a footballer and a midfielder. He also graduated from Sporting's youth academy, but spent most of his career in the lower leagues.As of 8 January 2015
Taça de Portugal: 2006–07
Supertaça Cândido de Oliveira: 2002
UEFA Cup: Runner-up 2004–05
Primeira Liga: 2009–10
Taça da Liga: 2008–09, 2009–10, 2010–11
UEFA Europa League: Runner-up 2012–13
Taça de Portugal: Runner-up 2012–13
Supertaça Cândido de Oliveira: Runner-up 2010
Toulon Tournament: 2001