Seth was born Gregory Gallant on September 16, 1962, in Clinton, Ontario, Canada. His parents were John Henry Gallant and the English-born Violet Daisy Gallant (née Wilkinson); he was the youngest of their five children. His family moved frequently, and he grew up mostly in Tilbury, Ontario. He was inward, unathletic, and had few friends, and took to comic books and drawing at a young age.
Seth attended the Ontario College of Art in Toronto from 1980 to 1983. He became involved with the punk subculture and began wearing outlandish clothing, bleaching his hair, wearing makeup, and frequenting nightclubs. He took on the pen name Seth in 1982.
As of 2004, Seth lived in Guelph, Ontario, with his wife Tania Van Spyk, whom he married in 2002.
