 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.After a failed campaign in 1910, Lonergan was elected as a Democrat to the U.S. House of Representatives from Connecticut's 1st congressional district in 1912, serving from March 4, 1913, to March 3, 1915.