Midfielder Merrifield and right-back Fenlon, both 18, have put pen to paper on new deals after completing two successful years with the Dons' academy.
Merrifield told the Wimbledon Guardian: "I've trained with the first-team a couple of times and can't wait to start full-time training in pre-season."
Fenlon added: "Head of youth Mark Robinson has helped me with my tackling and heading to make me a better player and hopefully I can do well training with the first-team and get a chance to show what I can do."
Dons boss Terry Brown added: "I'm pleased to announce we have signed two of our best young players.
"They will now come on board with the first-team squad during pre-season and hopefully they can play a part in League Two."
