Justman was born in Washington, D.C. He co-wrote many of the band's songs with singer Peter Wolf, and took sole songwriting credits for the band's biggest international hit, "Centerfold" (#1 for six weeks on the Billboard Hot 100). Justman also became the band's main vocalist once Wolf left the J. Geils Band in 1983 until the band's first break-up in 1985.
Seth is the brother of Paul Justman, director of the 2002 movie, Standing in the Shadows of Motown. Paul and Seth shared songwriting credits for most of the songs on J. Geils Band's 1984 album, You're Gettin' Even While I'm Gettin' Odd, which was also the one album featuring Seth on lead vocals.