In Westlife's music, Mark is distinctive for his rich, soulful voice, which is prominently featured in all of Westlife's songs. He shares most lead vocals with Shane Filan.
Before Westlife Mark, Shane Filan and Kian Egan came from the band IOYOU where he had partly written the band's songs, "Everlasting Love" and "Together Girl Forever".
Feehily has spoken to The Boston Globe of his embarrassment about Westlife's Rat Pack album Allow Us to Be Frank and blamed their record company for forcing them to record it after Robbie Williams had success with this type of music but dismissed Westlife as "a weird time in our career, really".
Mark has also co-written a few songs, along with the other band members, including:
- "Imaginary Diva"
- "Reason For Living"
- "Crying Girl"
- "You Don't Know"
- "Never Knew I Was Losing You"
- "Where We Belong"
- "Singing Forever"
- "I Won't Let You Down"
- "You See Friends (I See Lovers)"
- "I'm Missing Loving You"
- "Miss You When I'm Dreaming"
- "Reach Out"
- "From Now On"A
- "If That's What Love Is"A
A "From Now On" and If That's What Love Is" were found to be Feehily's co-written songs. No other details whether it's for Westlife or not.
Mark's parents are Marie and Oliver Feehily. He has two younger brothers named Barry Feehily (born 1985) and Colin Feehily (born 1989). 
Feehily publicly came out as gay in the UK tabloid The Sun, and announced his long-term relationship with Kevin McDaid, a member of the now-defunct British boy bandV. In an interview he said, "My life has got umpteen times better. My sexuality doesn't define me as a person, but it's a huge part of anyone to be open about it. It's changed my life totally as I feel so much happier and at ease. It's the best thing I've ever done." Unlike his band mates, for years Feehily did not discuss his private life. His manager Louis Walsh was unaware of Feehily's sexuality when he picked him for the band.
The pair have been dating since January 2005 after having met at a Cheerios Childline Concert in Ireland. In yet another interview, Feehily told The Sydney Morning Heraldthat despite the fact that the majority of Westlife's fans are female, there had really been no negative reaction. "I am so much happier now and I think our fans can see that," he said. "Through the years of getting to the point where I wanted to come out, I had to go through every kind of emotion, but I can honestly say that the day before I came out, I didn't care what the reaction was going to be. That was the hardest point to get to."
Feehily said that while he did not intend on getting married right away, he is happy to know he could form a civil partnership in Britain with the recent passing of the Civil Partnership Act. They both say that when the time is right, they will look into having a civil partnership ceremony.
In December 2007, Feehily and McDaid appeared on the cover of Attitude. They also did a joint interview. In this interview, Mark stated that he wants to grow old with Kevin, and that the pair believe that they will be together forever.
They had been a couple since January 2005. Mark and Kevin got engaged on 28 January 2010 and Feehily confirmed the news via Twitter.