Max Sargent is a business executive.
More specifically, he is an experienced and highly valued business Procurement Officer, considered one of the best in the U.K. "with twenty years' experience leading Buying departments for some of the world's largest companies".
We learn straight off how when Sargent was nineteen, he joined the Royal Marine Commandos, receiving the normal rigorous training that that esteemed military unit is known for. Then on his first deployment, a mission went very wrong very quickly and several of his unit were killed. Suddenly and not surprisingly, Sargent lost his interest in remaining with the military and headed to the corporate world.
He chose this line because as a teenager he had worked for his father's small plastics molding business and had learned how the "raw materials, equipment and sundries were being bought". Seeing the importance the cost of goods were to the bottom line, he also saw he had a knack for negotiations, getting better prices and terms and helping improve end profits. He could easily have stayed in that line after his A-levels (high school for us Yanks) but wanted something different than the office or the university, thus his foray into the Marines.
Now, when we first meet him, he is forty years old and described as "six-foot-tall, slim, dark hair and a discerningly handsome face". He is at the top of his game and would have likely stayed there for another twenty years or more but for one error in judgment that threw it all into turmoil.
That is when MI5 came to visit. They needed someone with his skill set and his experience and his integrity. Here then was a chance to make amends for his mistake and to have a chance to get his life back on track.
That is, of course, that this one little assignment did not get him killed.