Ex Don puts pen to paperMason will join the rest of the squad for fitness testing on June 14th, before the first day of pre-season three days later.
"We won't bring in players for the sake of it, they have to be quality, and Mason certainly fits the bill of what we look for in our full backs."
"He really impressed us with his attacking prowess from full back and he also has a great attitude, along with high levels of fitness."
U21s manager Hall told the official website: "We got a call to say that Mason was available so we asked if we could have a look at him in our U21s games.
Spence has spent the past two seasons in the youth team at MK Dons but was told he wouldn't be offered a contract at stadium:mk.
The eighteen year old full back put pen to paper on a year's contract, having impressed in the final few matches of the season for Richard Hall's U21s.
Mason Spence has signed for the U's.