Harold Clark graduated from Beat Four High School in rural Mississippi and played football at the University of Southern Mississippi. Clark was a two-time letterman at Southern Miss between 1955-56. He spent 32 of his 37 years as a high school head coach at Vigor High School in Prichard.

Clark took over as the head coach at Vigor in 1974. In his first season the Wolves went 0-10, but showed improvement the next year finishing 3-6. Clark continued to build the program into a winning team and in 1978 he led Vigor to a regional title, the first of his head coaching career.

In 1987, Coach Clark led Vigor to a State Championship which he repeated the following year. The 1988 Wolves went undefeated and is considered one of the greatest high school teams in Alabama high school history as they were named the National High School Champions by both USA-Today and ESPN.

Coach Clark remained at Vigor High School through the 1994 season. He ended his head coaching career with eight area and region titles and a region record of 63-36. During his career Harold Clark posted an overall record of 160-74-1, with only four losing seasons in 21 years. His playoff record stands at 27-10 with nine consecutive appearances between 1984 and 1992

Clark was also the athletic director at Vigor for twenty years. He was named Coach of the Year by the Alabama Sports Writers in 1994. Coach Clark is a member of the Mobile Sports Hall of Fame and was inducted into the Alabama High School Athletic Associations Hall of Fame in 1995.