If you look at current US coins, you can find D mint marks, P mint marks and S mint marks.

In te past, coins from philly would not carry a mint mark, but at some point in time, they started placing a P mint mark on the coins from teh Philly Mint.

However, not on the cent.

Any idea why on all coins except the cent?

It is some sort of a cost saving measure, apparently it cost them too much to put the P on the die for the cents. Quite a few cents have also been minted at West Point mint and San Francisco using the plain un mintmarked coins. This maybe due to the fact that people would save them if they had S or W mintmarks.

Back in 1980 when they started using the P mintmark, all of the dies had the mintmarks punched into them by hand. Now for the five cent through dollar adding he P to the dies this amounted to maybe 1,500 more dies to punch. But for the cent, with its much larger mintage, it was around 5,000 dies just for Denver. If they were to put the P on the cents as well that would be ANOTHER 5,000 dies to punch. So they just decided to leave the Philadelphia cent blank.

In 1985 they started adding the S mintmark to the model that the master hub is made from. Now all of the proof dies for a given denomination had the mintmark in exactly the same place. This worked so well that they started doing the same thing to the business strike dies and by 1990 all of the mintmarks were on the models. At this point they could have started using the P on the cents but instead they just made the D master hub and used it to make the D Mint Master dies. then they would grind the mintmark off the hub and use the same hub to make the Philadelphia master dies. This is where the 1997 "Phantom D" and 1998 "Phantom S" cents came from, an improperly ground master hub.

Thanks Ukraine Dvi and Conder101

Good info. I did not know that about the D mint marks. It makes sense now, though!