Martin has written songs for a variety of acts including Embrace (« Gravity ») and Jamelia (« See It in a Boy’s Eyes », co-written with Coldplay producer Rik Simpson). Martin has also collaborated with Ron Sexsmith, Faultline, the Streets, and Ian McCulloch. He also sang a part of the vocals for the Band Aid 20 single « Do They Know It’s Christmas? » at the end of 2004. In 2005, Martin collaborated with Nelly Furtado on the track « All Good Things (Come to an End) », for her 2006 album, Loose. The two were once rumoured to be a couple, after they both performed at Glastonbury in 2002. Nelly Furtado joked about it, saying, « Yeah, he’s my boyfriend — he just doesn’t know it yet ».

Martin’s fascination with hip hop was shown in mid-2006 when he collaborated with rapper Jay-Z for the rapper’s comeback album, Kingdom Come, after the two met earlier in the year. Martin put some chords together for a song known as « Beach Chair » and sent them to Jay-Z who enlisted the help of hip-hop producer Dr. Dre to mix it. Coldplay producer Rik Simpson conceived and performed the drum beats. The song was performed on 27 September 2006 by the two during Jay-Z’s European tour at Royal Albert Hall. Martin has also worked on a solo collaboration with Kanye West, with whom he shared an impromptu jam session during a 2006 concert at Abbey Road Studios. He performed the chorus of « Homecoming », from Kanye West’s album Graduation.

In 2015, Martin collaborated with DJ Avicii to work on two new tracks for his album Stories. Their first collaboration was officially named « Heaven ». Martin wrote the lyrics, Avicii did the production, and Simon Aldred of Cherry Ghost was the vocalist. He also provided the vocals for Avicii’s True Believer, also in Stories. In February 2017, Martin performed « A Different Corner » at the 2017 Brit Awards in tribute to George Michael.

A song he co-wrote called « Homesick » appears on Dua Lipa’s self-titled debut album, which was released in June 2017. In 2019, Martin was featured on Avicii’s posthumous album Tim. The song « Heaven » features vocals by Martin and was written by Avicii and Martin prior to Avicii’s death.