From wikipedia:
Dear Mr. Fantasy" is a rock song by Traffic off their 1967 album, Mr. Fantasy. An extended (10:57) live version of the song also appears on the 1971 Traffic album Welcome to the Canteen. It was written by Jim Capaldi, Steve Winwood, and Chris Wood, although Winwood took on most of the musical construction of the song and sang it on the album
The song has been covered by Canadian rock band Big Sugar in 1995, Jimi Hendrix and by Crosby, Stills, Nash and Young