Cassidy aiva mutsigiri uyo akatamba muOntario Hockey League (OHL) neOttawa 67 kubva muna 1982 kusvika muna 1985, uye akasarudzwa neChicago Black Hawks yeNational Hockey League (NHL) muna 1983 NHL Entry Draft
, akasarudzwa mumutambo wokutanga, wechi18 chose chose. Mwaka wakanakisa waCassidy weOHL waive muna 1982/83, paakanyora zvibodzwa makumi maviri neshanu uye makumi masere neshanu anobatsira mapoinzi zana negumi nerimwe. Akakunda Memorial Cup pasi pemurairidzi Brian Kilrea muna 1984 achiwana mapoinzi gumi nemaviri. Pazera remakore gumi nemapfumbamwe, akaita NHL yake yekutanga neBlack Hawks munaKurume 1984. Kubva muna 1984 kusvika muna 1988, akavhiyiwa ibvi katatu, kusanganisira kuvhiyiwa kwakakura kweACL yake.