Thornhill had a background in freelance journalism and publishing including working as a film critic.
He was a member of the WEA Film Study Group in the 1960s, where he met writers Ken Quinnell and Frank Moorhouse.
He has directed many films with screenplays written by Frank Moorhouse including The American Poet's Visit (1969), The Girl from the Family of Man (1970) and The Machine Gun (1971), Between Wars (1974), The Everlasting Secret Family (1988) and the docudrama Who Killed Baby Azaria?
His screenplays include The Esperance story (1968), Cheryl and Kevin (1974), and his feature film The Journalist (1979) .
He has been the producer for many of his films including Between Wars (1974), The F.J. Holden (1977), The Ever-Lasting Secret Family (1988) and Who Killed Baby Azaria?