After leaving Graham Parker completed legal studies at Adelaide University, later lecturing there and undertaking a Master of Law. He undertook further studies at Columbia University, New York City, where he obtained a second Masters and, following the awarding of a Ford Fellowship, a Doctor of Juristic Science.
Graham Parker taught at Vanderbilt University, Tennessee, before moving in 1965 to Osgoode Hall Law School at York University, Toronto. He was Professor of Law at York from 1973 and specialised as a teacher of criminal law and statutory interpretation. He also established and edited family law and legal history journals. Several of his students have become justices of the Supreme Court of Canada while others have collaborated with him in academic pursuits.
Apart from a 3 year term at the Australian National University, Graham Parker remained at York University until his retirement in 1996, by which time he was acknowledged as one of the foremost scholars of Canadian legal history.