McGregor, whose initial loan deal at Meadow Lane ended earlier this month, has been training with Notts and had stated his desire to extend his stay with the club.
However, interest from a number of other clubs meant that the deal wasn’t straightforward and Notts Chief Executive Jim Rodwell was full of praise for the way that McGregor’s parent club Celtic handled the negotiations.
“We are obviously delighted to have secured Callum’s signature for the rest of the season and I am sure that our fans will share in the excitement and anticipation of seeing him back in action for us,” said Rodwell.
“I have to say a big thank you to Celtic for the way that they have worked with us throughout not only the initial negotiations to bring Callum here in the first place, but also since his loan deal ended and in light of interest from a number of high profile clubs.
“It is understandable that Callum has been attracting the interest of other clubs because he is obviously a very talented player with a bright future ahead of him, but Celtic kept us informed at every stage of the process and were fair and honest throughout,” he added.
“Now we are just looking forward to seeing Callum pull on the black and white shirt once again and continue his development with us here at Meadow Lane over the coming weeks and months.”
McGregor goes straight into the squad for the Sky Bet League 1 match against Walsall here at Meadow Lane on Saturday.