Answer For dollar coins minted in 1935 and earlier, the mint mark is on the reverse, or "tails" side, of the coin, usually below the eagle. Many times the mint mark gets lost in the rest of the design. A poor strike or a well-circulated, well-worn coin may obliterate the mint mark, or make it otherwise unrecognizable.
Specific positions for the major designs:
For dollar coins minted in 1971 and later, including both the nickel and brass dollar coins minted for circulation, the mint mark is on the obverse, or "heads" side, of the coin, usually near the date. These coins are made of copper-nickel or brass so they're not "silver" dollars, although many people mistakenly call them that.
Bullion American Eagle silver "dollars" also have their mint marks on the obverse, although these coins carry an artificial denomination and are meant for investment, not use in commerce.
Mint mark letters for non-gold dollars (including the "golden" Sacagawea dollar) are as follows:
Any US coin without a mint mark was minted in Philadelphia, because for a while, it was the only mint.
The "P" Mint mark was not used on any US $1 coin until 1979. Your coin was struck in Philadelphia
No US 1799 coin will have a mintmark, all were struck in Philadelphia.
There's no "MD" mint mark on any US coin. The mint mark on a Peace dollar is near the word ONE on the back: No mint mark = Philadelphia D = Denver S = San Francisco Please see the Related Question for more.
If it has an S mint mark and a whitish color, it's made of copper and silver. If it has a D mint mark above the date or no mint mark (Phila.) it's actually made of copper and nickel. Please see the Related Question for more.
US Coins with no mint mark were minted at the US Mint's main facility in Philadelphia, Pennsylvania.
The "O" mint mark represents the US mint at New Orleans, Louisiana .
One Dollar
Any US coin without a mint mark was minted in Philadelphia, because for a while, it was the only mint.
If the coin has a mintmark, it's on the reverse above the letters DO in dollar.
The "P" Mint mark was not used on any US $1 coin until 1979. Your coin was struck in Philadelphia
No US 1799 coin will have a mintmark, all were struck in Philadelphia.
There's no "MD" mint mark on any US coin. The mint mark on a Peace dollar is near the word ONE on the back: No mint mark = Philadelphia D = Denver S = San Francisco Please see the Related Question for more.
If it has an S mint mark and a whitish color, it's made of copper and silver. If it has a D mint mark above the date or no mint mark (Phila.) it's actually made of copper and nickel. Please see the Related Question for more.
If it has one it's just above the eagles tail on the reverse. No mint mark = Philadelphia D = Denver S = San Francisco
On the reverse under the wreath. If there's no mint mark the coin was struck in Philadelphia.
$28 to $35,000 depending upon the mint mark and the condition of the coin.