Born into a working class home, Blethyn pursued an administrative career until her early 30s before enrolling in the Guildford School of Acting after the dissolution of her marriage in 1973.
Poleg tega je dobila nagrade BAFTA, Empire Award in zlati lev, pa tudi nagrado Theater World Award ter nominaciji za nagradi Critics' Circle Theatre Award in nagrado Laurencea Olivierja za svoje delo v gledališču.