Camden got into the game and started showing true quality, and two quick-fire goals put the home side ahead.
Dave Kanzen levelled the scores following good work from Rob Hershkorn who then set up Marks who put the Premier Division 2-1 ahead.
Mill Hill did not let their heads drop and when Ben Grabiner powered in a bullet header at the back post, it looked as though the tie would go to extra-time, but Marks had other ideas and his last-ditch effort broke Mill Hill hearts.
Relieved Camden manager Josh Marks told JC Sport: "We were weak defensively but Rob Hershkorn and Guy pulled us out of trouble. The only good thing really was the result because it was another below par performance."
"We’re going to need a lot more to get through to the finals – we’ve never played Hendon A before are so looking forward to it."
Mill Hill boss Joe Levy told JC Sport: "Full credit to Camden. They are a great team and are a pleasure to watch. Jake Saunders in the middle is superb and is definitely one to keep an eye on.
"I am absolutely gutted we are out but cannot fault the desire and commitment of any of my players. Hopefully we keep this level of performance going for the rest of the season."