Mimic began permanently displaying the original X-Men's powers in X-Force (1st series) #45, but how exactly he started to do so remains unknown.
Mimic's status as a mutant has changed over the years. Initially, he was a non-mutant whose powers came from a chemical accident in his father's lab. However, Marvel Comics Presents #59 suggested he was actually a latent mutant whose abilities were triggered by the accident. In Dark X-Men: In the Beginning #1, further testing confirmed he was NOT a mutant, only mimicking mutant abilities. As of Immortal X-Men #10, though, he has been resurrected on Krakoa and serves as a back-up for the Resurrection Five. This possibility was mentioned as far back as House of X #5. Since Judgment Day, resurrection has been made available to non-mutants to some degree, so his status remains ambiguous.