Lonergan was born in Hobart in Tasmania in March 1899,[1] and was educated at State High School in Launceston.[2] He joined the Australian armed forces during World War I and saw action the Battle of Gallipoli aged only 16 and later in France, where he was badly injured.[3] He then served in the Australian army headquarters in London.[3] After the war he studied at the Repatriation Trades School in Launceston from 1920 to 1922,[2] before joining the civil service in the Territory of New Guinea in 1923. He married Norfolk Islander Irene Mitchell in February 1929.[2]
In 1940 he was appointed Assistant Government Secretary.[2] During World War II he was part of the Australian New Guinea Administrative Unit, achieving the rank of lieutenant-colonel and was mentioned in dispatches.[3][4] He became Acting Government Secretary in 1949,[2] and in 1951 took on the role permanently.[5] As a result, he was appointed to the Legislative Council in 1952.[6]
The Secretary Department was scrapped in 1955 and Lonergan became Director of Civil Affairs. He was also Acting Assistant Administrator for five months in 1956–57.[7] He retired in 1959 and returned to Australia. He died in Sydney in 1969, survived by his wife.[3]