Bonus answer: You say that the mermaids evolve from a fully mammalian creature, but you don't state if it is the same ancestor as humans evolved from. So there are two possible answers:
As Sumelic stated they would probably have no tails. In that case the fishtail would probably evolve from the legs. Because of the way evolution works, creatures adapt their bodies by the way they use them. If you don't have a tail, you would use your legs to swim, so through the course of evolution the legs would evolve into the fishtail. It would look mostly like the tail of a seal. Keep in mind that the arms would also evolve over time, so you should find a reason why they didn't turn into flippers. They need their hands for some reason.
If they don't have the same ancestor and this ancestor has a tail. Then the tail would probably evolve much in the same way as a dolphin or whale.
Overall I think the first option is the most likely one, since mermaids have arms and a face much like a human has.