Born in Belfast, Cassidy began his career at Glentoran before joining the Magpies in October 1970. The skilful Northern Ireland international established himself in the United side and starred in the club's runs to Wembley for both the 1974 FA Cup final and 1976 League Cup final.

Cassidy spent ten years at St. James' Park, making a total of 239 appearances in a black and white shirt, scoring 28 goals. He left Tyneside for Burnley in July 1980, spending three years with the Clarets before heading to Cyprus for the final two seasons of his playing days, which he spent with APOEL.

After making his debut for his country in the year following his move to Newcastle, Cassidy went on to win 24 caps for Northern Ireland, scoring once, and was part of their squad for the 1982 World Cup in Spain.

After retiring, he enjoyed a lengthy career in management, which included spells in charge of APOEL, Gateshead, Glentoran, Ards and Sligo Rovers. He later managed Workington, Newcastle Blue Star, Whitby Town and Blyth Spartans.

In recent years, Cassidy suffered with Alzheimer's disease, but was a regular attendee at the club's Memory Café, a drop-in session at St. James' Park for those living with life-changing memory loss illnesses.

The thoughts of everyone at Newcastle United are with Tommy's family and friends.