 During his time at Parramatta High School, a priest from St. Felix's parish discouraged Cassidy from becoming a priest because he had not finished his secondary education, had not studied in Catholic schools and his family background was definitively "unsuitable", due to financial difficulties after his grandfather died in 1939.He worked at the New South Wales Department of Road Transport as a junior clerk, having to stop his schooling.In 1942, he went directly to Archbishop Norman Thomas Gilroy of Sydney to present his case for entering the priesthood; Archbishop Gilroy agreed and he entered St. Columba's College (seminary), Springwood, in February 1943.