His second season was somewhat mixed. With new manager Sam Allardyce preferring to play Michael Owen and Mark Viduka up front, Martins found himself being used as an impact substitute for the first half of the season. Despite this he managed to score consistently, and when Allardyce was sacked and Kevin Keegan was appointed, Martins found himself being used as part of a three pronged attack along with Owen and Viduka. Martins scored less goals in his second season with Newcastle, mainly due to him no longer being the focal point of all their attacks as he was in his first season with them, as well as his season being interrupted with the African Cup of Nations participation with Nigeria in December.

Martins scored 6 goals in 12 league games during the first half of the 2008–09 season before a torn hamstring ruled him out for several games. It was confirmed on 7 January 2009, that Martins would undergo hernia surgery in Germany the next Monday. He was be operated on by renowned specialist Dr. Ulrike Muschaweck. He did make it back in time for the last two games of the season, scoring in a relegation six-pointer derby against Middlesbrough. However, he could not stop Newcastle being relegated to the Coca-Cola Championship. He initially reiterated his desire to stay and help the club return to the Premier League for the 2009-10 season, but he eventually left for VfL Wolfsburg. His last game was on a preseason friendly against Leyton Orient F.C., which Newcastle lost 1-6.

During his three-year spell on Tyneside, Martins became a firm favourite of the fans and amassed 39 goals in all competitions from 105 games.

On 29 July 2009 Newcastle accepted a £9million bid from Bundesliga champions Wolfsburg, the transfer was confirmed on 31 July with Martins signing a four year contract. The four year deal was finalised on August 12, with the club officially unveiling the Nigerian international to the press and Wolfsburg supporters'. For his debut, Martins came on as a second half substitute against FC Koln at the Rhein Energie Stadion, with his team deadlocked at 1-1. He helped in the buildup to the second goal and capped off his first appearance in the Bundesliga for Wolfsburg with a well-taken goal after dribbling past two on-rushing defenders and then cooly finishing from six yards out, slotting the ball past the Cologne keeper to the far post.

On 9 July 2010, Martins was sold for a reported €8 million to reigning Russian Premier League champions Rubin Kazan. He signed a three-year contract and told BBC Sport: "I am joining a team of champions and I hope to bring my best to the club to make them even greater." Martins' agent, Nicola Walter, revealed that Rubin met all of the player's requirements and that manager Kurban Berdyev had a "massive role" for him to play, following the departure of striker Aleksandr Bukharov. Martins will be able to make his debut with Rubin after August 1, when the transfer window is opened.

Martins has scored seventeen goals in 32 appearances for Nigeria. He was a part of the Nigerian team that finished third in the 2006 African Cup of Nations. Martins scored two goals in the competition, both coming against Senegal in the first round.

His future for the national team has been placed in doubt on numerous occasions after separate incidents of controversy. In August, 2006 the Nigerian FA website mistakenly had his date of birth down as 1 May 1978. This would have made him twenty-eight years of age, rather than twenty-one. The Nigerian FA soon confirmed that it had been an administrative error and apologised for the confusion it had caused. However Martins threatened to quit playing for the national team after the blunder had initially put his move to Newcastle in doubt.

On 6 February 2007, he did not turn up for the 4–1 friendly defeat against Ghana, despite being ordered to play. He later explained his absence was due to the fact he had travelled to Lagos in order to visit his sick mother. The Nigerian caretaker coach Augustine Eguavoen was furious with Martins and threatened to ban him from the national team. On 14 February 2007, Martins apologised for missing the game but said he believed his mother's health was more important. He also said that he will be available to play for Nigeria in the future. He made his return to the national side on 24 March 2007, playing the full match against Uganda in an African Cup of Nations Qualifying match and won.

Following the end of Nigeria's 2008 Africa Cup of Nations qualification campaign, Nigeria faced Mexico in Ciudad Juárez for a friendly match. Martins scored two goals in a 2–2 draw.