All British Pennies from 1883 to 1911 inclusive have no mintmark.
They were all minted at the Royal Mint London.
There is no mintmark on the 1921 British Penny, they were all minted at the Royal Mint London.
US or British, post new question.
If there is a mintmark at all, it will be a very small "H" to the left of the date in the exergue. The "H" indicates it was minted at Heaton in Birmingham. All 1912 Pennies without a mintmark were minted at the Royal Mint.
I presume that you are referring to a circulated US 1931 cent with no mintmark under the date, rather than a British penny. The answer then is almost certainly under a dollar.
British 1875 Pennies were minted at either the Royal Mint London, or the Heaton Mint in Birmingham. The Royal Mint Pennies have no mintmark. The Pennies minted at Heaton have a small "H" below the date on the reverse. All British Pennies minted at Heaton from 1874 to 1882 inclusive will have the same mintmark in the same place.
A mintmark is a small identification mark used to tell which mint it was made in.
It will have a S mintmark.
underneath the year
Assuming you mean a U.S. Indian Head cent and not a British penny, 1900 is a common date for Indian Head Cents. In average circulated condition it's worth $1.00-$3.00.
1900-D is not a valid date/mint for a U.S. penny. All pennies in 1900 were made at the Philadelphia Mint, so have no mintmark. If your coin has a "D" on it, it must have been added after the coin left the mint. 1900 Indian Head Cent values : heavily worn -- $1.00 medium worn -- $2.00 lightly worn -- $7.00 uncirculated -- $20.00 to $25.00
An Australian Penny or Halfpenny with a "Y." (PENNY.) mintmark indicates that the coin was minted at the Perth Mint. An Australian Penny or Halfpenny with a "I" mintmark indicates that the coin was minted at either the Calcutta or Bombay Mints in India.
The English/British One Penny coin has been issued for over 1,100 years. All years of issue of the British decimal New Penny and Penny are still potentially in circulation so, unless they are part of a Proof or uncirculated mint set or are individual Proof or uncirculated coins and in absolute mint condition, they are worth 1 Penny. 20th century predecimal Pennies would need to be in mint condition to attract any significant value. The most spectacularly valuable example would be the 1933 Penny. There are 7 known coins. Other than that, there would be the - 1919 (KN mintmark & H mintmark), 1918 (KN mintmark & H mintmark), 1909 (variant - 1 directly above tooth) 1902 (variant - with low tide - further done Britannia's shin)