He is the lead singer and frontman of the Southern rock band Black Oak Arkansas, in which he is the sole remaining original member.

While in high school, Mangrum and classmate Rickie Lee Reynolds formed a band named The Knowbody Else.

In 1970, they traveled to Los Angeles where they signed with Atco Records and released their self-titled first album with their new name Black Oak Arkansas.

Black Oak Arkansas: Underdog Heroes was released May 24, 2019, under Cleopatra Records featuring Sammy B. Seauphine (who has been in the band since 2014 and is Jim Dandy's business partner) and Shawn Lane.

Featuring Sammy B. Seauphine, Tim Rossi formerly of Blackoot, and Kinley "Barney" Wolfe formerly of The Cult and Lord Tracy.