A condition of perfection or newness suggestive of that of a freshly minted coin.
This expression has nothing to do with the aromatic plants which derive their name from their genus Mentha. To 'mint' is to form a shape by stamping metal and this is the straightforward source of 'mint condition'. The phrase is used to describe items that are in brand new condition and is often applied to things that are old or delicate and might be expected to be showing some signs of wear. An example of such a usage is found in the earliest citation of the phrase that I have found in print - from the Scottish newspaper The Evening Telegraph, October 1895:
A Mauritius post paid 2d blue, unused, with original gum, fine margins all round, and in mint condition, realised £140.