Harwell was born in Florence, South Carolina.
He received a Bachelor of Arts degree from Clemson University in 1980 and a Juris Doctor from the University of South Carolina School of Law in 1982.
He was in the South Carolina Army National Guard from 1987 to 1992, and was a law clerk to Judge G. Rodney Peeples of the South Carolina Judicial Department in 1983, and to Judge G. Ross Anderson of the United States District Court for the District of South Carolina from 1983 to 1984.
On January 20, 2004, Harwell was nominated by President George W. Bush to a seat on the United States District Court for the District of South Carolina vacated by Charles Weston Houck.
He served Chief Judge from February 28, 2019 to June 4, 2024 when he assumed senior status.