Gillis was born in White Plains, New York. He grew up in suburban Westchester County, and attended Pocantico Hills Central School and Sleepy Hollow High School. In 1969, he entered the University of Chicago, where he earned a BA, an MA, and achieved "ABD" status ("All But Dissertation").

Gillis' first work in the comics industry was as a freelance writer for Marvel Comics, where he worked on such titles as Captain America, Marvel Two-In-One, and Super-Villain Team-Up from 1978-1980. Gillis then worked as an editor for the Florida-based publisher New Media Publishing; he left that position in 1981.

Gillis is probably best known for Shatter (1985—1988), the first digital comic; and the entire run of First Comics' Warp (1983—1985). Gillis co-created Morituri (1986—1988) with artist Brent Anderson. Gillis wrote the entire runs of The New Voyages (1984—1986) and Strange Tales vol. 2 (1987—1988); other Marvel comics Gillis wrote include numerous issues of What If...? vol.1 (1980—1984), The Defenders vol. 1 (1984—1986), and The Eternals vol. 2 (1985—1986).

His more eccentric inventions for other companies include Blaze Barlow and the Eternity Command and the Black Flame for First; and the gonzo post-holocaust comedy Gammarauders (a tie-in to the Gamma World role-playing game) for DC Comics' short-lived TSR Games line. For DC Comics' he also wrote the critically acclaimed science fiction miniseries Tailgunner Jo with art by Tomosina Artis.

Gillis now lives in Elgin, Illinois.