Ku qor sii socodka faqradan - Cassidy wuxuu ahaa difaaca oo ka ciyaaray Ontario Hockey League (OHL) oo uu la ciyaaray Ottawa 67's 1982 ilaa 1985, waxaana doortay Chicago Black Hawks ee National Hockey League (NHL) ee 1983 NHL Entry Draft
, waxaa lagu soo xulay wareega koowaad, kaalinta 18-aad. Xilli ciyaareedkii ugu fiicnaa ee Cassidy ee OHL wuxuu ahaa 1982/83, markii uu diiwaan geliyey 25 gool iyo 86 caawin ah 111 dhibcood. Wuxuu ku guuleystay Memorial Cup tababarka Brian Kilrea 1984 isagoo dhaliyay 12 dhibcood. Markii uu ahaa 19 jir, wuxuu ku soo biiray NHL markii ugu horeysay ee uu la ciyaaray Black Hawks bishii Maarso 1984. Laga soo bilaabo 1984 ilaa 1988, wuxuu lahaa seddex qaliin oo jilibka ah, oo ay ku jiraan dib u dhiska weyn ee ACL.