For eight years, Mitchell worked for MI6, five years of which he spent as the personal bodyguard of MI6 head M. However, unbeknownst to everyone around him including those he worked with, Mitchell was a double-agent for the secretive Quantum terrorist organization.
In August 2006, James Bond captures Mr. White, a high-ranking member of Quantum, and escorts him to Siena, Italy where M, Mitchell and agent Kim Park are waiting to interrogate him. Mitchell excuses himself to check the perimeter, where he secretly murders a guard stationed outside. Upon his return, Mr. White remarks that his organization has "people everywhere" at which point, without warning, Mitchell draws his gun and quickly kills Park before unsuccessfully trying to shoot M. Mitchell and Mr. White both escape, but Bond pursues Mitchell into the midst of the Siena Palio Horse Race.
Trying to get away from Bond, Mitchell fires his gun behind him several times, possibly killing an innocent bystander. Bond eventually chases Mitchell all the way to the top of a bell tower, where the two get into a struggle and fall through some glass onto some scaffolding in an adjacent building undergoing reconctruction. They both lose their weapons and race to retrieve their guns, but Bond is slightly quicker and he snaps around first, shooting Mitchell in the head and killing him.
During MI6's subsequent examination of Mitchell's apartment, they find some marked bank notes connecting him to geologist Edmund Slate, which inspires Bond to travel to Haiti in search of Slate. Among Mitchell's personal affects are an ashtray and three other Christmas presents M had bought for him, suggesting a close working relationship between the two of them.
Mitchell appears in the 2008 video game adaption of Quantum of Solace. This version of Mitchell has the first name Henry as opposed to Craig, and rather than being shot by Bond, he is killed when Bond throws off of some scaffolding and he falls to his demise. He is voiced by American voice actor Fred Tatasciore.