Hannigan received his law degree from Saint Louis University. From 1933-1942, he was chairman of the Democratic City Committee of St. Louis and was the chief aid to Mayor Bernard Dickman. After working as collector of Internal Revenue for the Eastern District of Missouri and as commissioner of Internal Revenue, he was chosen as the chairman of the Democratic National Committee. He served in this capacity from 1944 to 1947. During this time, he was also postmaster general of the United States and was an unofficial advisor to President Truman. In 1947 he and Fred Saigh bought the St. Louis Cardinals baseball team. He sold his interest to Saigh in 1949, the year he died.