The United States Mint was created in Philadelphia in 1792. The Philadelphia Mint eventually joined with several other US Mints, each of which has, or had, their own mint mark.
Currently, there are four US Mints that produce coinage, located in Philadelphia, Denver, San Francisco and West Point. Historically, other US mints included the New Orleans Mint, the Dahlonega Mint, the Charlotte Mint and the Carson City Mint (which served as a branch mint).