McManus is the ill-tempered Irish left-side gunner of the "Black Bess". Mistrustful, cynical, and highly vocal, he verbally chastises the new and inexperienced driver Edwards upon their meeting, and again when he stalls the tank after taking the controls for the first time - each time he has to be pacified by Finch and Townsend. His pessimism is likely a result of his extended involvement in the war, as he as known to have been witness to countless offensives. Based on remarks made on McManus' predicted non-participation upon the promise of "wine, women and song" waiting for the crew in Cambrai, it is possible he is married or does not drink. He is also seen to be religious - a rosary and crucifix hanging by his weapon station, as well as his performance of the sign of the cross when he believed he was about to be killed by an artillery strike, denotes him as a Catholic Christian.
When Black Bess gets stuck in the mud and overran by infantry, Townsend attempts to call in artillery fire on their position using a carrier pigeon. McManus, believing this to be completely suicidal, desperately tries to browbeat Edwards into not releasing the pigeon, but fails as Townsend pulls rank. While waiting for the strike, he sarcastically declares that it was "Nice serving with you lads". He is surprised when the barrage saves the tank, but continues his tirade, lamenting that their luck will run out eventually. He is silenced by Townsend. Soon after, when a severed fuel line starts pouring petrol into the crew compartment, he uses a rolled-up towel to mend the broken lines and stop the leak before the trio drives on.
Black Bess fights through a foggy forest before breaking down just short of the rally point the following dawn. The crew soon discover the rest of their unit has been decimated and their machines salvaged by the Germans. McManus and Edwards are ordered into the village of Bourlon to retrieve new spark plugs for the tank, but once outside, McManus again voices his belief that the mission is futile, citing the deaths of Finch and Pritchard. After an argument in which Edwards suggests he should abandon the crew since that's what he wants, McManus storms off by himself, leaving Edwards to go into enemy-held Bourlon alone. He is not seen again until Edwards returns with the spark plugs and is attacked by a bayonet-wielding German. McManus saves Edwards by killing the enemy, before apologizing for his behaviour, remarking "I lost my way back there". The two reconcile, and Edwards mentions nothing of their falling out to Townsend.
From that point on, McManus seems much more enthusiastic and committed to the mission. He laughs when Edwards finally gets the engine working again and said that Bess likes Edwards when he swears, and appears less anxious as they battle through large numbers of enemy tanks on their way to the railway station. He also seems to finally accept Edwards as one of the crew, referring to him as the driver and appearing concerned for his well being.
After defeating a major mechanised counterattack, the triumphant Black Bess is immobilized by artillery while on the home stretch to Cambrai. Edwards leaves the tank to start repairs just as a German infantry charge envelopes the machine. McManus goes to assist him after he is blasted off his feet by a grenade bundle, shooting several Germans in his defense before being shot himself in the back. Black Bess is destroyed, with Townsend sacrificing himself to kill the crew's attackers, leaving Edwards and a wounded McManus as the sole survivors. After being helped to his feet, McManus asks his fellow crewman for orders. McManus, supported in his step by Edwards, limps towards Cambrai.