He graduated from Manhattan College in 1949 with a degree in labor relations. Donahue's union career actually started a year before that when he became a part-time organizer for the Retail Clerks International Association. From 1949 to 1957, he held several positions with Local 32B, the flagship local of the Building Service Employees International Union (BSEIU), including business agent, education director, contractor director, and publications editor. Meanwhile, he attended night classes at Fordham Law School and received his law degree in 1956.