Long joins the Sky Bet League One club on a youth deal and will remain at stadium:mk until February 8.
The 18-year-old has been brought in after leading scorer Patrick Bamford's loan spell ended at the weekend, with his parent club Chelsea preferring his development to continue at Championship level with Derby.
"I've known Chris since he was seven and he's somebody who has always been high on our list of potential targets," Dons boss Karl Robinson told the club's official website.
"He's only young at 18 years of age but he's talented, he's quick and I know he's somebody who the FA speak very highly of.
Long will be available for the Dons' league fixtures fixtures against Shrewsbury, Bristol City, Crewe, Preston, Carlisle, Tranmere and Walsall.
However, the England Under-20 international will not be eligible for the FA Cup replay with Wigan as he was not registered for the initial tie on Saturday. He has, though, been given clearance to play in the fourth round should the Dons make it past the holders.
Long progressed through the Everton youth ranks before being offered his first professional deal last July. This year the speedy forward, who has yet to make a first-team appearance for the Toffees, has scored five goals in 11 Under-21 Premier League matches.
- Sports & Recreation
- Patrick Bamford
- Karl Robinson