After retirement from playing, Cassidy became a manager, taking charge of Ards, Sligo Rovers, Workington, Whitby Town and Blyth Spartans.
Although Cassidy's Football League debut did not take long to arrive, he made his first appearance in a 2–0 defeat at Southampton on 7 November 1970, he had to wait three seasons to establish himself in the Newcastle first eleven.
The following season Cassidy was troubled by injury and made just nine League appearances, though he did play in the first leg of the Texaco Cup Final as Newcastle retained the trophy 3–1 on aggregate against Southampton.
Cassidy's Northern Ireland career had begun at the end of his first season in English football, coming on as a substitute for Eric McMordie in a 1–0 Home Nations defeat by England.
He did however return to favour for Blanchflower's last few matches, starting in a 5–1 defeat by England at Wembley, and coming off the bench during a 1–0 victory over the Republic of Ireland at Windsor Park.
Bingham used Cassidy in this role for each of his first nine games in charge, thus allowing the likes of McIlroy, Brotherston, Finney and O'Neill to make a suitable attacking impact.
He played an important role in Northern Ireland's 1980 Home Nation Championship success, helping bring the old trophy back to Belfast for the first time in 66 years.
A £30,000 move to Burnley, freshly relegated to Division Three, in the summer of 1980, did not initially harm Cassidy's international career, but over the following few seasons it became clear that Bingham was favouring players featuring regularly at a higher level.
In November 1981 he was called up to replace Martin O'Neill for the final World Cup Qualifier against Israel, a must win game to ensure a place in Spain.
In Spain he made a single appearance, coming off the bench for the injured Sammy McIlroy, and helping to steady ten-man Northern Ireland, during the historic 1–0 win over the hosts.
Sligo just missed out on promotion the following year when they lost the final match of the campaign 4–1 to Home Farm, and Cassidy left the club that summer [2].