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" 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.