He was born on a farm near Newington, Ontario, the son of William Milligan, an Irish immigrant. He taught school and then attended Toronto University and Osgoode Hall. He was called to the bar in 1899 and set up practice in Cornwall. Milligan served on the high school board, was a police magistrate and then juvenile court judge. In 1928, he was named King's Counsel. Milligan married Maud Percival.