He was mayor of Glace Bay from 1913 to 1915 when he enlisted in the Canadian Expeditionary Force fighting in World War I.

Harrington became Minister of Labour in the government of Edgar N. Rhodes and became Premier of Nova Scotia when Rhodes left provincial politics to enter the federal cabinet in 1930.

During Harrington's term he was able to end ongoing labour disturbance among miners in Cape Breton which had afflicted the previous two premiers.

He improved the provincial department of mining, fought for the coal and steel industries in Ottawa, and passed legislation calling for a national policy on coal and steel.

However, his government was unable to combat the Great Depression and was defeated by the Liberals in the 1933 election.