Cassidy was a member of Liverpool's FA Youth Cup-winning team of 1996 but unlike some of his colleagues in that team he was unable to make the big breakthrough at Anfield and was released by Liverpool before he made a first-team appearance. The closest he came was being named as one of the five substitutes for a home Premier League match with Sheffield Wednesday in December 1996. Cassidy moved to Cambridge United in 1999 when he was 21 years old but only played in a handful of first-team matches. He subsequently moved back to the north-west to play for Northwich Victoria in the 2000/01 season and later appeared for Unibond League Burscough. In March 2024, Cassidy was jailed for 13 years and three months for his part in a conspiracy that saw 356kg of cocaine with an estimated street value of £28million flood cities in north of England.