Marcus McManus was a Starfleet physician serving during the 24th century.
He graduated Veterinary School and practiced for two years from his family's farm but wasn't satisfied. Going through an arduous process he was accepted into the Starfleet Medical Academy. After graduation he applied to serve in the Fleet Marine Force, providing medical treatment to Marines in the field.
As a freshly minted Lieutenant Junior Grade, he reported to the USS Normandy as a surgeon attached to the 11th Marine Expeditionary Unit's CASH. The ship left for a seventh-month presence patrol of the Federation's Demilitarized Zone border. The Marine unit participated in several training scenarios, including planetary invasion, ship boarding and space station seizure, and other evolutions intended to let the Cardassians know they were ready to act on any hostile actions. The ship also visited Prophet's Landing and two other Bajoran colonies.
The team was on hand to provide medical aid when refugees fled the Dominion's massacres in the Demilitarized Zone.
When hostilities with the Dominion broke out the 26th Marine Expeditionary Unit and was on the frontlines of the Dominion War. The constant death wore him down, and by the end of the war, he was drinking himself to sleep each night.