"A career soldier, McCullin has had any hope he may have once harbored beaten out of him. Now he knows death is the only way he'll ever get out of the war."
— Call of Duty 3 bonus materials section.
Sgt. McCullin was born in Chicago, and became an American soldier who fought in Saint-Lô and led many times during the battles his team faced. On the first mission, McCullin regrouped with his squad in Saint-Lô and led his squad through the town and almost shot Guzzo for him trying to leave the squad, Dixon going against this. Guzzo says that McCullin isn't crazy enough to shoot him, but McCullin says that desertion is a capital offense and that it would be the sanest thing he's ever done. After taking the town, a second wave of German troops comes towards the building, then McCullin and his squad is forced to defend a key building from impeding German troops and a Tiger tank. Soon after, he was transferred to the 90th division, and helps take down German defensive bunkers all over St. Germain. Soon they are pushed forward into a town near the Mayenne river. Sgt. McCullin wanted to get out of the war and he thought the only way was to die. Sgt. McCullin's wish came true when he was mortally injured while trying to defuse some bombs on a bridge on the Mayenne River. After disarming the last bomb, McCullin begins to slowly die while Dixon runs to his side. His last words were "Tell Guzzo... tell him... tell him to go to hell." He then dies laughing with joy.
McCullin's Helmet is notably battered, has a slightly darker hue than the other helmets, and it has the symbol for the 29th Infantry Division painted on its front, even while he is in the 90th Infantry Division. The helmet's notable, because it is passed down from soldier to soldier when the former dies, and the latter is promoted to Sergeant. After McCullin dies, Dixon wears the helmet in his time as Sergeant, and Guzzo is seen wearing it at the ending cutscene after Dixon is killed, where he is seen telling the new arrivals about McCullin's "one rule".
McCullin was an engineer at one point, according to Dixon.
It could be possible that McCullin started to show signs of "shell shock" when crossing the river while being bombarded by extremely accurate mortar fire. This symptom affected many of the soldiers in World War I and World War II. This could have also lead to his "suicide", as patients often became suicidal.
McCullin had a huge amount of hate toward Guzzo as his last words where "Tell Guzzo go to hell..."
Dixon and McCullin's friendship and actions may have been the basis for the characters Sgt. Roebuck and Sgt. Sullivan from Call of Duty: World at War. McCullin and Sullivan both die making their corporal (Dixon and Roebuck) sergeant. These two then die at the end of the American campaign (Roebuck dying is up to the player
McCullin's character model.
McCullin addressing his squad at the start of the game.