[3] Cassidy worked as a journalist before entering political life, and was bureau chief of the Financial Times in Ottawa for a period.

Focussing on provincial politics, Cassidy was re-elected with an increased majority in the 1975 election, in which the NDP under Stephen Lewis reduced the Conservatives to a minority government and became the official opposition in the legislature.

The NDP fell back to third place, behind the Liberal Party, in the 1977 provincial election, and Lewis resigned as leader the following year.

Cassidy's policy advisor in the leadership campaign was James Laxer, a former leader of The Waffle NDP faction which had separated from the party in 1974.

Cassidy faced a difficult re-election in Ottawa Centre, defeating PC candidate David Small by only 599 votes.

He campaigned for the federal New Democratic Party in the 1984 election, defeating Progressive Conservative candidate Dan Chilcott by 54 votes to win the Ottawa Centre riding; two-term Liberal MP John Leslie Evans fell to third place.

Cassidy opposed the Harris government's plan to restructure and partially privatize the Crown corporation, and remained a director until 1997.