Lonergan was born in Thompson, Connecticut, to a father from Ireland and a Canadian mother of Irish descent.

He was admitted to the bar in 1901 and practiced law in Hartford, Connecticut.

He was a members of the city planning commission and was assistant corporation counsel of Hartford from 1910 to 1912.

He was elected to the Senate in 1932, narrowly defeating Hiram Bingham III amidst the Democrat landslide that year, and served one term from March 4, 1933, to January 3, 1939.

Lonergan practiced law in Washington, D.C. until his death on October 18, 1947.