Rubuta ci gaba don wannan sakin layi - Cassidy ya kasance mai karewa wanda ya taka leda a gasar Ontario Hockey League (OHL) tare da Ottawa 67's daga 1982 zuwa 1985, kuma Chicago Black Hawks na National Hockey League (NHL) ta zaba shi a cikin 1983 NHL Entry Draft
, wanda aka zaba a zagaye na farko, na 18 a duka. Kyakkyawan lokacin OHL na Cassidy ya kasance a cikin 1982-83, lokacin da ya yi rajistar kwallaye 25 da taimakawa 86 don maki 111. Ya lashe Kofin Tunawa a karkashin kocin Brian Kilrea a 1984 inda ya ci maki 12. A lokacin da yake da shekaru 19, ya fara buga wasan NHL tare da Black Hawks a watan Maris na shekarar 1984. Daga 1984 zuwa 1988, an yi masa tiyata sau uku, ciki har da sake gina ACL.