Those larger leaves look more like a sage/salvia than any mint. The smaler ones definitely look like mint. Have you tried crushing a smelling them to compare? Mint is unmistakeable, while salvia - if that's what it is - can have a kind of skunky smell.

Anyway, if the large leaves don't smell mint-y, you know it's not a mint.

Where are you located? Here in FL, those leaves might also be beach sunflower or similar.

Thanks. Yes, it seemed unlikely that it was mint. There is actually a big patch of horseherb nearby, and I'm betting that's what it is. That stuff doesn't propagate will from seeds, but it would have been in the compost that was added to the mix.