His childhood was uneventful, happy... normal. His dad was in the police, he was a traffic officer. Mark joined the City of London Police in 1990 and then he was transferred to the Metropolitan Police. He worked on some very interesting stuff such as investigating race and hate crimes against minority groups.
In the mid 90's there were lot of operations on preventing street level drug dealing and lot of them were undercover. He passed the course and shortly afterwards he was accepted onto a team. He was a test-purchase officer, part of a whole investigative team usually based on particular London estate. Basically he'd go and buy drugs from a suspected dealer and other people would be photographing the event and watching his back and latter on arrests would be made.
With practice he adopted a new personality, he opened up lot of doors, he met lot of different people involved in crime, and he broadened up his horizons. He was so good that one time when a dealer was arrested, and when they told her that she sold drugs to an undercover cop, she wouldn't believe it... she was keep saying "no way, he's a friend of mine."
Soon, the National Public Order Intelligence Unit got interested in him. He didn't even know that such unit existed. He was invited to a couple of meetings and interviews and he was finally selected. He was told that his role will be to provide information to the authorities so they could proportionally police demonstrations.
Confessions of an Undercover Cop - Full documentary