The 25-year-old prop joined Edinburgh from Yorkshire Carnegie last year, afer impressing Richard Cockerill with his performances.
He made his debut for the Capital club in the 1872 Cup against Glasgow Warriors at Scotstoun and was drafted to Heriot’s for the season.
Speaking to Ealing’s website, Millar Mills said he was looking forward to returning to London and getting started at Vallis Way.
The former London Scottish tighthead said: “I’m really excited for the challenge of the Championship and moving back down to London.
“I’ve watched and played against Ealing Trailfinders as they’ve moved up the leagues and I’ve always been impressed with the set up here.”
Ealing’s Director of Rugby Ben Ward added: “Elliot already has Championship experience and his set-piece is very strong so we’re confident he will add plenty of quality for next season.”