Where is the mint mark on US coins?

To quote a famous phrase, "it's not that simple". There is no single position for a mint mark on a U.S. coin. Positions vary depending on the coin's denomination and date, and most Philadelphia coins minted before 1980 don't have a mint mark at all.

In general coins made before 1965 had their mint marks somewhere on the back, usually near the bottom, but some (e.g. cents) had the mint mark on the front under the date, and others (standing Liberty quarters) put the mint mark elsewhere on the front.

During 1965-67 mint marks weren't used because the mints were cranking up production of the then-new cupronickel clad coins. Starting in 1968 most mint marks were moved to the front side near the date - but again, not always.

Letters to look for on modern coins:

• P = Philadelphia. Dollars 1979-present; nickels, dimes, quarters and halves 1980-present; nickels 1942-1945.
• D = Denver. All denominations 1906-present
• S = San Francisco. All denominations up to 1954; dimes and cents 1955; various cents and nickels 1968-74; dollars 1979; all proof coins 1968-present EXCEPT 1999 dollars.
• W = West Point. Modern gold and silver commemoratives only; collectors' strikes of 1996 dimes.

Obsolete mint marks:

• O = New Orleans
• CC = Carson City
• D = Dahlonega, GA (up to 1861, no confusion with Denver)
• C = Charlotte

So you see the situation is REALLY variable. I'd suggest you determine what denominations and dates you're interested in first. Then check a site such as coinfacts.com that shows the mint mark position (often with pictures) for every denomination and date of U.S. circulating coin since the 1830s.