Harry Coyne was elected to the Queensland Legislative Assembly at the 1908 election in the electoral district of Warrego. He held the seat until he retired on 31 July 1923. During that period, he was Chairman of Committees from 15 July 1915 to 12 October 1916, Secretary for Railways from 13 October 1916 to 30 April 1918, Secretary for Public Lands from 22 October 1919 to 12 November 1920, briefly Minister without Office from 12 November 1920 to 16 December 1920, and then again Secretary for Public Lands from 16 December 1920 to 2 July 1923.
Harry Coyne was the chairman of a number of subcommittees of the Queensland War Council including those concerned with Anzac Cottage Trust and the commemoration of Anzac Day.
He was also involved in the construction of Anzac Avenue to Redcliffe. On 7 June 1926, Harry Coyne was the passenger in a taxi that hit an electric light pole outside the Excelsior Hotel in Flinders Street, Townsville.
He was found lying in the street unconscious and was taken by ambulance to hospital where he was found to have a fractured skull. Without having recovered consciousness, Harry Coyne died on 12 June 1926 surrounded by his family.
The taxi driver Otto Korn was also hospitalised but survived.
He was a Member of the Queensland Legislative Assembly.