Why do some pennies not have mint marks?

Answer 1

Some pennies do not have mint marks because they were produced at the Philadelphia Mint, which is the main mint facility for producing US Coins and does not place mint marks on its coins. Prior to 1980, all pennies minted at the Philadelphia Mint did not have mint marks. Therefore, if you come across a penny without a mint mark, it is likely from the Philadelphia Mint.

Answer 2

Actually, since 1955 about half of all US cents don't have a mint mark. The coins are made at the Philadelphia and Denver mints in roughly equal quantities.

Philadelphia has never put its "P" mint mark on cents even though it's been on all other denominations since 1980. The reason is that sometimes demand for pennies can be so high that Philadelphia and Denver can't keep up even working 24/7. When that happens one or both of the 2 other mints at San Francisco and West Point are pressed into service as well.

Those 2 mints normally only make collectors' coins and can't produce anywhere near as many cents as Denver and Philadelphia. If there were small numbers of S- and W-mint pennies they'd be seen as "instant collectibles" and would disappear from circulation right away. Instead, the Mint's directors decided that cents from Philadelphia, San Francisco and/or West Point would all be "plain" (i.e. no mint mark). The coins get mixed together in circulation and no one can tell the difference.

The backstory: Philadelphia never used a P mint mark on most coins up till 1980. The reasons are a bit complex. When the US first started making coins Philadelphia had the only Mint so a mint mark wasn't considered necessary. When other mints opened they did use mint marks but by then it was considered traditional for Philadelphia coins to be "plain". In fact, up till 1979 only one issue of Philadelphia coins ever had a P mint mark: during WWII nickels were made out of a copper/silver/manganese alloy to save metal for the war effort. A "P" was used from 1942 to 1945 to indicate those coins made from the new alloy.

The P disappeared from nickels after the war ended, so all Philadelphia coins were "plain" once again. Finally in 1979 the Mint's directors decided to make mint marks consistent and began using the P mark on $1 coins that year and other denominations the next year, except for pennies.