Did the US Mint put mint marks on every coin?

No. In fact, there are no mint marks on coins made in Philadelphia prior to 1979, except for the famous "war nickels" issued from 1942 to 1945. The first US mint marks were used in 1838 when branch mints were opened in New Orleans, Dahlonega, and Charlotte.

During the changeover from silver to clad coinage mint marks weren't used on any coins dated 1965-67.

Starting in 1979, $1 coins gained the P mint mark. The following year all other Philadelphia coins except for cents also began using the P mint mark.

Philadelphia cents continue to be struck without mint marks, for complex reasons having to do with special production at other mints when Philadelphia and Denver can't meet the demand for pennies.