For Stewart, the band's success became a decidedly mixed blessing. He found the frenzied young fans who mobbed him and his bandmates on the U.S. tour to be harrowing, even terrifying; additionally, he and the other members had seen some success as songwriters on the B-sides of two of their singles, and he had become much more interested in pursuing that side of music than touring the world with a guitar or two in tow. He also resented the group's shift from its original R&B sound to pop ballads, which was the direction that Fontana and the record label wanted to go in. The Mindbenders parted company with Fontana in late 1965 and continued to work on their own, Stewart taking over as lead singer. But despite some great albums, plus an appearance in the hit movie To Sir with Love, they weren't able to sustain their career momentum, and by 1968 they were forced to accept offers to play the cabaret circuit, the last refuge of bands on the decline. He finally announced his intention to leave, which ended the group.

It was during the summer of 1968 that Stewart received an offer from producer/manager Peter Tattersall to invest in Inner City Studios, a small recording operation in Stockport, where Stewart had already been recording demos. He duly joined the company as a partner, and they soon took over larger quarters, which Stewart renamed Strawberry Studios, drawn from the Beatles song "Strawberry Fields Forever." A new partner joined later in the year when Graham Gouldman, a successful songwriter (and an ex-member of the Mockingbirds, as well as the last bassist for the Mindbenders), came aboard. It was soon after this that Stewart was contacted by producer Giorgio Gomelsky about playing lead guitar for a duo he was working with, consisting of Lol Creme and Kevin Godley. Stewart and Gouldman began offering the pair regular session work at Strawberry, and soon there were four musician/partners at work there.

In December of 1969, in order to generate some income and develop the studio's facilities, they signed a deal with American producers Jerry Kasenetz and Jeff Katz, whose Super K Productions specialized in pop-oriented "bubblegum" music. Stewart, Godley, Creme, and Gouldman worked for three months, writing and releasing songs as the Ohio Express, Crazy Elephant, and a brace of other names owned by Super K. Stewart, Godley, and Creme also subsequently generated a two-million-selling single ("Neanderthal Man") of their own under the alias Hotlegs, released by the Pye label. They also ended up touring with the Moody Blues which, unfortunately, led nowhere in terms of future work.

It was a series of sessions for Neil Sedaka that convinced the four -- Gouldman having rejoined after a stay in America working for Super K on his own -- that they had a special sound together as a band, and led to the formation of 10cc. They immediately took the rock press by storm and, over a period of four years, from 1972 through 1976, were among the most widely written-about pop/rock outfits in England, releasing a brace of hit singles and albums that sold around the world. The quartet didn't last past that point, Godley and Creme exiting in 1976, though Stewart and Gouldman became a very effective and successful songwriting team. Starting with the breakup of the group, he also established himself as a producer, working with Justin Hayward and John Lodge of the Moody Blues (most notably their single "Blue Guitar"), Neil Sedaka, and Agnetha Faltskog in her post-ABBA period. A car crash in early 1979 nearly derailed his fledgling solo career, though he did recover and has gone on, in between producing other artists' work, to release three solo albums in the 30 years since 10cc's demise. A fourth has reportedly been promised for sometime after 2008.