Gene Day began his career in the Canadian alternative comix scene. In 1974, he published the short-lived underground comic Out of the Depths. He cooperated with Dave Sim on Oktoberfest Comics #1, published by Now and Then Publications in 1976. Day drew for the Skywald magazines Psycho and Nightmare from 1974, as well as Mike Friedrich's independent comics company Star Reach, contributing to Star Reach anthology, Image and Quack. Day did illustrations for fantasy role-playing games and published his own graphic novel, 'Future Day', in 1979.
He was a longtime inker on Marvel's 'Master of Kung Fu' title by Mike Zeck, starting in 1976. He also inked on the licensed 'Star Wars' series, pencilled by Carmine Infantino, as well as 'The Mighty Thor' and 'Marvel Two-in-One' featuring 'The Thing'. From 1985 to 1986, Renegade Press published four issues of Gene Day's 'Black Zeppelin', an anthology series primarily featuring stories and painted covers Day completed before his death, as well new contributions by Sim, Bruce Conklin, Augustine Funnell, and Charles Vess. More of his work appeared posthumously in Caliber Comics' anthology series Day Brothers Presents, which also featured the work of Day's artist brothers, David Day and Dan Day.