Affleck wrote, directed and stars as a young criminal ready to change his life in the new movie - his second foray into directing since his critically-acclaimed 2007 debut Gone Baby Gone.
And starpower from Jeremy Renner, Rebecca Hall, Jon Hamm and Blake Lively helped the film take in a winning $23.8 million (£15.9 million) in earnings.
At a gala screening of The Town in London on Sunday (19Sep10), Affleck told Upbeat Productions' Filmbeat: "Reaction (to) the film has been great, I was shocked and it seems to be doing very well. I can't tell you (how) exciting and thrilling it is, ya know? We are the number one at the box office in the U.S. this weekend... so imagine my surprise!"
Affleck's film beat competition from high school comedy Easy A, which debuted with $18.2 million (£12.1 million) in takings, while new thriller Devil debuted at three with $12.6 million (£8.4 million).
Milla Jovovich's action-packed Resident Evil: Afterlife followed at four with $10.1 million (£6.7 million), while animated family film Alpha and Omega - featuring the voices of Christina Ricci, Justin Long, Hayden Panettiere, Dennis Hopper and Danny Glover - rounded out the top five by raking in $9.2 million (£6.1 million).