Mills is credited with being the chief composer behind many of R.E.M.'s songs, including "Nightswimming",  "Find the River", "At My Most Beautiful", "Why Not Smile", "Let Me In", "Wendell Gee", "(Don't Go Back To) Rockville",  "Beat a Drum", "Be Mine", “Electrolite”, and "What's the Frequency, Kenneth?".
In particular, R.E.M.'s 2004 album Around the Sun was heavily shaped by Mills' piano and keyboard contributions.

Mills is also responsible for the prominent backing vocal and harmony parts found within the band's back catalogue, with his vocal contributions arguably being most noticeable on 1986's Lifes Rich Pageant and 2008's Accelerate.
In addition to providing backing vocals, he has also sung lead vocals on the songs "Texarkana", "Near Wild Heaven", The Clique cover "Superman" and The Troggs cover "Love is All Around".

R.E.M. had been talking about disbanding since 2008, and eventually did so in 2011.
Since then, Mills said there is no possibility of an R.E.M. reunion.
Mills described it as "31 wonderful years and the opportunity to end it on our own terms."

Mills continues to write music and perform with friends on various projects.
In 2012, Mills contributed piano playing to a Record Store Day single released by Drive-By Truckers member, Patterson Hood, in protest of a Walmart development being built in Athens, Georgia.

Mills is a member, along with Steve Wynn, Scott McCaughey, Peter Buck, and Linda Pitmon, of The Baseball Project.

Mills also performs as part of singer-songwriter Joseph Arthur's band.
On April 3, 2014, while performing with musical guest Joseph Arthur, Mills broke the news that David Letterman would be retiring in 2015.
Mills took a band self-portrait that he posted to Instagram and did a short interview about "breaking" the story
Since 2010, Mills has played with a rotating group of musicians for a series of concerts built around Big Star's seminal album, Third/Sister Lovers.
  Known as Big Star's Third, the concerts have taken place in London, Sydney, Chicago, Seattle, Los Angeles, and New York.

A longtime Big Star fan, Mills wrote the liner notes for the 2014 reissue of the band's first two releases, 1972's #1 Record and 1974's Radio City.

In 2016, he toured to support a Concerto for Violin, Rock Band, and String Orchestra with childhood friend Robert McDuffie.
Mills is credited with being the chief composer behind many of R.E.M.'s songs, including "Nightswimming",  "Find the River", "At My Most Beautiful", "Why Not Smile", "Let Me In", "Wendell Gee", "(Don't Go Back To) Rockville",  "Beat a Drum", "Be Mine", “Electrolite”, and "What's the Frequency, Kenneth?". In particular, R.E.M.'s 2004 album Around the Sun was heavily shaped by Mills' piano and keyboard contributions.
Mills is also responsible for the prominent backing vocal and harmony parts found within the band's back catalogue, with his vocal contributions arguably being most noticeable on 1986's Lifes Rich Pageant and 2008's Accelerate. In addition to providing backing vocals, he has also sung lead vocals on the songs "Texarkana", "Near Wild Heaven", The Clique cover "Superman" and The Troggs cover "Love is All Around".
R.E.M. had been talking about disbanding since 2008, and eventually did so in 2011. Since then, Mills said there is no possibility of an R.E.M. reunion. Mills described it as "31 wonderful years and the opportunity to end it on our own terms."
Mills continues to write music and perform with friends on various projects. In 2012, Mills contributed piano playing to a Record Store Day single released by Drive-By Truckers member, Patterson Hood, in protest of a Walmart development being built in Athens, Georgia.
Mills is a member, along with Steve Wynn, Scott McCaughey, Peter Buck, and Linda Pitmon, of The Baseball Project.
Mills also performs as part of singer-songwriter Joseph Arthur's band. On April 3, 2014, while performing with musical guest Joseph Arthur, Mills broke the news that David Letterman would be retiring in 2015. Mills took a band self-portrait that he posted to Instagram and did a short interview about "breaking" the story
Since 2010, Mills has played with a rotating group of musicians for a series of concerts built around Big Star's seminal album, Third/Sister Lovers.   Known as Big Star's Third, the concerts have taken place in London, Sydney, Chicago, Seattle, Los Angeles, and New York.
A longtime Big Star fan, Mills wrote the liner notes for the 2014 reissue of the band's first two releases, 1972's #1 Record and 1974's Radio City.
In 2016, he toured to support a Concerto for Violin, Rock Band, and String Orchestra with childhood friend Robert McDuffie.
Mike Mills