In 1996, Mostyn received his Bachelor of Arts (B.A.) in Philosophy from the University of Western Ontario. During his time as an undergraduate, Mostyn was a member of the Dean's Honour List, as well as a recipient of the Honour W Award. Graduated from the University of Western Ontario Law School in 1999, Mostyn was awarded the Douglas May Memorial Award, and delivered a paper at the 14th BILETA Conference: "Cyberspace 1999: Crime, Criminal Justice, and the Internet" at the College of Ripon and York St John, York, England while in his third year of law school.