When Did the U.S. Start Using Mint Marks on Coins?

By GovMint :

The U.S. began using mint marks on coins following a Congressional Act signed on March 3, 1835, which established mint marks and authorized the creation of the first branches of the U.S. Mint. These branches included Charlotte, Dahlonega, and New Orleans, all of which are no longer operational today. Mint marks are small letters on coins that indicate at which facility the coin was produced, such as 'C' for Charlotte or 'O' for New Orleans. Before the 1835 act, Philadelphia was the only national mint, so mint marks were unnecessary.

Sources Used:

https://www.usmint.gov/learn/collecting-basics/mint-marks