Cassidy had played
junior football for
Vale of Clyde before appearing as a trialist for
Celtic in a match against
Hibernian on 16 October 1912 at the age of 18, being registered under the surname of Smith for the match. His performance impressed enough for the club to offer him a contract the same day and was handed his first-team debut six months later on 15 March 1913, playing in a 1–0 defeat to
Motherwell. He featured rarely for Celtic in his first few seasons and instead served loans at several clubs to gain experience (including brief spells at
Abercorn,
Kilmarnock and
Clydebank plus more than a season with
Ayr United). Following the outbreak of
World War I, Cassidy enlisted in the
Black Watch (Royal Highlanders) 1/7th Fife Battalion, and was awarded the
Military Medal in November 1918. Cassidy scoring the winner to ensure the
Bhoys claimed their first Scottish Cup win since 1914. His eleven goals in a single Scottish Cup campaign remains a joint club record,
Jimmy McGrory (his understudy and then successor in the squad) having equalled the tally in the 1924–25 season. During his time there, Cassidy suffered from a severe bout of
influenza that caused him to lose 22lbs in weight and resulted in him struggling with his stamina levels for several years. He remained with Bolton for one season before joining
Cardiff City in 1925, also spending one year at
Ninian Park before returning to Scotland with
Dundee in an exchange deal that saw
Sam Irving move in the opposite direction (coincidentally Cardiff won the
FA Cup the year after Cassidy departed, as had also been the case with Bolton a year earlier). After two seasons with Dundee and one with
Clyde, in 1929 Cassidy moved to Ireland, joining
Ballymena, helping the club to victory in the 1929
Irish Cup final, defeating
Belfast Celtic 2–1. He later joined
Dundalk for the 1931–32 season but returned to Scotland in January 1932 with
Morton. ==International career==