"You know, I could've just missed."
"With your ego, not a chance."
As with most members of the Red Cell team, much of Mick's past is unknown, besides the fact that he is British and from Wales (and he's quick to clarify that he prefers to call himself Welsh), United Kingdom. He is also a skilled sniper. In The Fight, Prentiss suggested that he may be a former member of the British Special Forces. Mick is a close friend of team leader Sam Cooper. He doesn't elaborate on how they met, but simply says that they "bumped into each other" a few times in the past. He also said that he would trust Sam with his life and, if it should ever come to that, die for him.
During the investigation of the murders committed by John Vincent Bell, Mick is paired up with Prentiss, whom he sometimes playfully flirted with. Together, they figure out what he did to his victims and, at one point, patrol a street together looking for Bell picking up more victims with Mick providing sniper support. When both BAU teams and SWAT raid the building Bell used to hold his victims captive, they eventually corner him on a rooftop. He initially pretends to commit suicide by falling from an edge. When Prentiss looks down for him, however, he turns out to have landed on a secure surface. Raising his gun, he is then shot by Mick using an SVD sniper rifle.
Sometime later, before being called out to a case in Alaska, Prentiss is shown debating with JJ about whether or not to ask Mick out, or just get a cat. In later episodes, she does have a cat, a black one named Sergio.