During this time, he was a prep All-American player under head coach Walter Shublom and was named consensus first-team all-state as a junior and senior as he led Wyandotte to back-to-back Class AA state championships in 1964 and 1965.

During his sophomore year, Allen played on the varsity team and averaged 15.5 points per game while helping lead the Bruins to an undefeated 30–0 season and the 1967 National Championship.

During his junior year, he averaged 15.1 points per game and helped the Bruins win a second consecutive NCAA Championship while playing alongside Abdul-Jabbar.

As a member of the 1971 Milwaukee Bucks team, which also featured Alcindor, Allen earned an NBA championship ring.

This was Allen's last season in Milwaukee, during which he arguably played his greatest professional game, a 39-point and 6 assist effort during a loss against the Detroit Pistons on January 2.