Marshall is married to Catherine, a native of Galveston, Texas, and has three children, Emory, Ross and Elissa.

He also served as a Senior Inspector in the Office of Professional Responsibility, as the Deputy Regional Director for the Latin America Region, as Country Attache in Brazil, as the Resident Agent in Charge in Austin, and as a Special Agent in Dallas, Texas and Houston, Texas.

He headed DEA's worldwide air operations from 1986–1995 as Special Agent in Charge of the Aviation Division.

In this capacity, he directed international investigations which resulted in the arrest of important figures in major Mexican and Colombian drug organizations.

During Marshall's tenure, DEA also established demand-reduction programs and enforcement programs against ecstasy (MDMA) which was being used at record levels by young people in the U.S. Operation Red Tide dismantled the largest international ecstasy trafficking organization in the world.