The riders in the field which included Lance Armstrong tried to prevent the sprinters having their day but inside the final kilometre, Cervélo's Heinrich Haussler seized the moment and went.
The line was almost in sight but up came Cavendish to snatch the win before the line.
Speaking to www.cyclingnews.com after the race, the 23 year Colombia High Road rider said: "I wanted to prove I am more than just a sprinter, but a great rider - that is what I did today. "When Haussler went it took us all by surprise. I waited for a moment and then I had to close the gap along the barrier. It was touch and go, I did not know if I was going to get him,"
Cavendish cried when he was on the podium and said to Cycling News Gregor Brown: "I have just won a Monument. It is my first Monument I have ever ridden. I won Milano-Sanremo; it is one of the races you watch growing up. I took a lot of pleasure by winning it when a lot of people had written me off.
"There are three moments when I cried after winning. When I won the World Championships on the track for the first time; my first stage in the Tour de France and today. Everything had to go right on those days, and it did. All of them were different, but they all worked out."
Roger Hammond finished 92nd with Bradley Wiggins 108th, Jeremy Hunt 133rd. Ian Stannard did not finish.