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 is very vocal about his love for Norwegian new wave/Synthpop band a-ha. In 2005 he stated the following in an interview: "I found myself in Amsterdam the other day and I put a-ha's first record on. I just remembered how much I loved it. It's incredible songwriting. Everyone asks what inspired us, what we've been trying to steal from and what we listened to as we were growing up—the first band I ever loved was a-ha." Martin has also performed live together with Magne Furuholmen of a-ha, introducing him as "the best keyboard player in the world". In November 2011, he stated that "back when we didn't have any hits of our own we used to play a-ha songs."
Coldplay performed R.E.M.'s "Nightswimming" with Michael Stipe during their Austin City Limits performance in 2005, as a part of the Twisted Logic Tour. Martin went on to call "Nightswimming" "the greatest song ever written". He has called Richard Ashcroft, formerly of the Verve, "the best singer in the world". He also admires the lyrics of Morrissey.
PETA named him the World's Sexiest Vegetarian in 2005. However, he began eating meat again after his split with Gwyneth Paltrow. Martin is a supporter of Exeter City. According to an article released by Quartz in May 2018, Martin has an estimated wealth of £94 million.