Windsor Smith produced his first published work in 1967-68 - single page "Powerhouse Pinups" of Marvel Comics characters for Terrific and Fantastic comics, titles published by Odhams Press that included licensed Marvel Comics reprints for the UK market. Following this, he flew out to the U.S.A. in summer 1968 to the offices of Marvel Comics with fellow artist Steve Parkhouse. "I sent material first, and based solely upon a pleasant return note from Stan's assistant Linda Fite, my pal and me were at Marvel's doorstep in the blink of an eye." Largely due to his Jack Kirbyesque style, Marvel Comics Editor Roy Thomas gave him the job of drawing both the cover and story of an issue of X-Men, "The Rage of Blastaar" (X-Men #53, February 1969), credited to Barry Smith as he was then known. Without a studio base, and having been evicted from his hotel, he was forced to do much of the work sitting on park benches. Nonetheless, the resulting pages secured Windsor-Smith further work with Marvel, in Daredevil #50-52 (March–May 1969), a western short story, "Half Bree" (probably the story "Outcast" eventually published in Western Gunfighters #4, February 1971), and issue #12 of Nick Fury, Agent of S.H.I.E.L.D. (May 1968), both scripted by Steve Parkhouse. Windsor-Smith later called his early art "amateur and klutzy" and a "less than skillful" Kirby imitation, but Stan Lee liked it enough to give him more work. However, with his visa having expired and without a work permit, Windsor-Smith was sent home by U.S. Immigration Authorities in December 1968.
Despite this setback, Roy Thomas had sufficient faith in him to give him the art chores on issues #66 and #67 of The Avengers (July–August 1969) after he had returned to the U.K., and he continued to work at a distance for Marvel Comics for several years, providing the art for a number of stories in the horror anthology titles Tower of Shadows and Chamber of Darkness. Heralding things to come, Thomas, a long-time Conan the Barbarian fan, also had Windsor-Smith provide the art for a sword and sorcery story in the Conan style, "Starr the Slayer", in Chamber of Darkness #4 (April 1970). Soon after, Roy Thomas offered Windsor-Smith the job as the original penciller for Marvel's adaptation of Robert E. Howard's Conan the Barbarian character, starting with the first issue "'The Coming Of Conan", in Conan the Barbarian #1 (October 1970). In 1971, Windsor-Smith moved to the U.S.A., having been granted a work permit.
During his run on Conan the Barbarian, Windsor-Smith was involved in the writing as well. He and writer Roy Thomas adapted a number of R.E. Howard short stories, the aforementioned "The Frost-Giant's Daughter", "Tower of the Elephant", "Rogues in the House", and "Red Nails", the last of which was issued as a very limited, pirated, A3-size black-and-white version on glossy paper in England in early 1974. As well as the art and story contributions, Windsor-Smith provided the covers for most issues. They also worked on original adventures and characters based on R.E. Howard's characters - most notably the flame-haired warrior-woman, Red Sonja - loosely based on a character from one of Howard's non-Conan stories, who has now become a major comics character in her own right - in "The Song Of Red Sonja" in Conan the Barbarian #24 (March 1973), Windsor-Smith’s last issue of the title. By then he had worked on 21 of the first 24 issues of the series, missing only issues #17 and #18, and #22 (which was a reprint of issue #1), and both he and the title had won a number of awards. Windsor-Smith would later say that the reason he missed those issues was because he had quit the series a number of times as he was dissatisfied with the work and how the comics business worked, rather than the deadline problems Marvel quoted.
Windsor-Smith also provided the art for a number of other Marvel Comics titles, including the Ka-Zar stories in Astonishing Tales #3-#5 (December 1970-April 1971) and #10 (February 1972), three further issues of The Avengers (#98-100, April–June, 1972) - about which he would later remember the nightmare of drawing "all those bloody characters that I didn't give tuppence about", Iron Man #42 (June 1972), and Marvel Premiere #3-4 (July–September 1972), which featured Doctor Strange, both of which were apparently re-scripted by Stan Lee after being drawn to Lee’s original scripts. Windsor-Smith was by now becoming disillusioned with the comics industry and the way in which in his opinion the writers and artists were being exploited: "I needed to be free of constraints and policies that were imposed by the dictates of creating entertainment for children." Shortly thereafter, Windsor-Smith left comics for the first time, leaving only a couple of inventory items in the Marvel Comics vaults, both stories of R.E. Howard characters: Kull in "Exile of Atlantis" (Savage Sword of Conan #3, December 1974), and Bran Mak Morn in "Worms of the Earth" (Savage Sword of Conan #16, November 1975). Other than ten pages of inking of Jack Kirby pencil work for Captain America’s Bicentennial Battles (1976), a one-off oversize Marvel Comics Treasury Edition, he produced no more comics work until 1983.
At this point he changed his professional surname to Windsor-Smith, adding his mother's surname to his own, and began to pursue a career in fine art. Granted residential status in the United States in 1974, Windsor-Smith, along with his partner Linda Lessman, set up Gorblimey Press, through which he released a small number of limited-edition prints of fantasy-based subjects that proved popular. In 1976 Windsor-Smith published The Gorblimey Press Catalogue, a high quality index to the work published by Gorblimey Press, with full-page reproductions of each piece. Prior to that, in 1975, together with Jeff Jones, Mike Kaluta and Bernie Wrightson, he was one of four comic book artists-turned-fine-illustrator/painters who formed a small artist’s loft commune in Manhattan known as The Studio, with the aim of pursuing creative products outside the constraints of comic book commercialism. By 1979 they had produced enough material to issue an art book under the name The Studio, which was published by Dragon's Dream (ISBN 9063325819).