Mann studied at Southern Methodist University, where he was twice named to all-conference football teams and was nicknamed the "Little Red Arrow."

He subsequently worked his way through Harvard Law School; first with a job at a garment factory, later as a minister at a Congregationalist church.

After returning to Texas, Mann worked as an assistant attorney general under James V. Allred.

His opponents included Lyndon B. Johnson, then a member of the United States Congress, and Governor Pappy O'Daniel, who won the seat.

After resigning as Attorney General, Mann resumed private law practice in Dallas.