Sessions was born as John Gibb Marshall in Largs, North Ayrshire, Scotland. He changed his name to John Sessions when he registered at Equity. He attended the University of Wales, Bangor and graduated with an MA in English literature. He studied for a Phd from MacMaster University, Ontario, Candada, but he didn't complete the doctorate.
He also trained at the Royal Academy of Dramatic Arts.
He has an older brother and a twin sister.