Quiet Sun originally formed around Dulwich College in 1970 as a four-piece progressive rock band featuring Phil Manzanera (guitar), Bill MacCormick (bass), Charles Hayward (drums), and Dave Jarrett (keyboards). The band name came from an article on sunspots and solar flares that MacCormick's brother Ian had been reading, called "The Year of the Quiet Sun." The group managed to score a rehearsal space from Warner Bros. and toured around the south of England gaining a small following, but were never signed. In 1971, Manzanera left to join Roxy Music, MacCormick joined Matching Mole, Jarrett went into teaching, and Hayward joined Gong.

Three years later, Manzanera was riding high on Roxy Music and decided to reform Quiet Sun while spending the latter part of 1974 working on a solo album to be known as Diamond Head. Manzanera booked 12-hour studio days, recording his solo album for eight and Quiet Sun for four, resulting in Mainstream. The group mostly re-recorded the songs they had rehearsed back in 1970, though several songs from that time instead found their way onto Diamond Head, including "Frontera" (later re-recorded by Robert Wyatt as "Team Spirit" on Ruth Is Stranger Than Richard) and "Alma." Both albums formed the basis of much of 801's live shows, which featured Manzanera and MacCormick. Hayward left to form This Heat.

For a while, after initial pressings of Mainstream went out of print, the CD of the album was only obtainable through an expensive Japanese import. In 1999 Mainstream was re-released on Manzanera's own label, while a number of the early, pre-Roxy Quiet Sun demos were released in 2000 on Manzanera's Rare One. ~ Ted Mills, All Music Guide