He was briefly a member of the Claudy Young Unionist Association until April 1965 when he joined the Northern Ireland Labour Party.
[2] Committed to non-violence, he became a major figure in the Northern Ireland Civil Rights Association, which campaigned for equality during the late 1960s.
In 1968, Cooper resigned from the Labour Party and founded the Derry Citizens' Action Committee (DCAC),[3] serving as its president until the following year.
On 12 August – the start of the few intense days of violence which have become known as the Battle of the Bogside – Cooper tried to restrain Catholics protesting an Apprentice Boys of Derry parade by linking arms with John Hume and Eddie McAteer.
[6][12][13] Cooper organised a civil rights and anti-internment march for 30 January 1972, which was to develop into Bloody Sunday, in which fourteen unarmed civilians were murdered by soldiers from the Parachute Regiment on duty in Derry, who opened fire on the crowd.
[citation needed] In 1983, Cooper stood aside after the boundary changes for the new Foyle constituency to let his colleague and friend John Hume contest the seat.