Marcus was born on March 12, 1948 in Oklahoma City, Oklahoma. He enlisted in the Army on November 20, 1967 and attended boot camp at Fort Polk, Louisiana. 

Volunteering for Vietnam from the First Special Forces Group on Okinawa, he landed at Cameron Bay in 1969. Marcus was assigned as a Radio Operator with MACV-SOG at Command and Control South encoding and passing messages for the unit. As a Green Beret radio operator, Marcus supported highly classified reconnaissance operations and gathered intelligence deep in denied areas during the secret war. Serving with the 1st Exploitation Company, he crossed the fence into Cambodia and took part in dangerous cross-border missions conducted behind enemy lines. Marcus served among the recon men of MACV-SOG, whose operations in Cambodia, Laos, and North Vietnam were later recognized with the Presidential Unit Citation for extraordinary heroism and combat achievement. 

Marcus retired in 2008 after serving 37 years with the US Army.