The pacey 18-year-old, who can play in midfield or as a striker, goes straight into Saturday’s squad for the Evo-Stik Division One North trip to Mossley.
Manager Steve Kittrick said he is pleased to have been able to bring Flint to Queensgate.
“I’m Glad Niall is here. He’s very quick and he’ll add some pace to the side,” Kittrick told the Scarborough News.
“The lad has got good pedigree having been involved with the first team at Shrewsbury Town and then AFC Telford, and he’s a committed character.
“He’s still based in Telford at the moment, so he drove three hours and 20 minutes to train with us in midweek, then all the way back again.
“I signed him for Telford when Shrewsbury released him, and I thought that he would be the future of that club, so I’m sure he’ll do well here.”
Flint joined Boro’s former Evo-Stik South rivals Market Drayton Town on a short-term loan deal in November, but has been out of action since the festive period.
“The last time Niall played was Boxing Day, so he’s be a bit short on match fitness at the moment,” Kittrick added.
“That said, he’ll be involved at Mossley and we’ll see how he goes.”
Follow @SN_Sport on Twitter for Martin Dowey’s live updates from Boro’s clash at Mossley, 3pm kick-off on Saturday.