William Allain graduated from the University of Mississippi School of Law in 1950 and established a law practice in Natchez shortly thereafter. He served in the United States Army infantry during the Korean War and resumed his legal career upon his honorable discharge in 1953. Allain held the position of Assistant Attorney General of Mississippi from 1962 until 1975, during which he represented the state in various courts, including the United States Supreme Court. He continued practicing law in Jackson until 1979, demonstrating extensive experience in both state and federal legal systems.

William Allain's political identity is marked by a blend of progressive legal advocacy and a pragmatic approach to economic issues. His involvement in landmark cases like the integration of the University of Mississippi and the Conner v. Johnson reapportionment cases showcases a commitment to civil rights and equitable representation. Despite his progressive legal stances, Allain's legislative record reveals an unexpected convergence with opposing factions on environmental regulations, hinting at a highly calculated approach to protecting regional industrial interests. His military service and active participation in veterans' organizations underscore a deep-rooted respect for service and sacrifice, which may influence his policy decisions.