Captain Beyond is unique among predominately guitar driven hard rock albums in the fact that it contains a wide range of rock, heavy metal, and jazz influences often with various time signatures and broad range of dynamics within the same song. The first five songs are fairly straightforward, but the remainder of the album consists of two suites of tightly arranged interconnected songs. The first starts with "Thousand Days of Yesterdays (Intro)" and ends with "Thousand Days of Yesterdays (Time Since Come and Gone)". The second starts with "I Can't Feel Nothin' (Part 1)" and finishes the album. Songs flow directly into each other without any lag time between selections, a feature that is shared with other more progressive bands of the era such as Moody Blues and Jethro Tull.
All of the songwriting was credited to lead vocalist Rod Evans and drummer Bobby Caldwell; however, according to Caldwell from a 2010 interview, all the songs were written and arranged by Evans, Larry "Rhino" Reinhardt, and himself, with some input from Lee Dorman. Due to their still binding contracts with Iron Butterfly, Reinhardt and Dorman could not be listed as songwriters on this record for legal reasons.