The 24-year-old has signed a pre-contract agreement with Ronny Deila's side and will join up with the Bhoys in the summer.
And Celtic supporters appear pleased with the quality of their new signing. The Dundee United winger has pace and trickery in abundance (as verified by Hoops defender Adam Matthews yesterday) and will no doubt add depth to Deila's squad.
But many fans are asking, however, whether the SPL champions really needed to wait until the summer to recruit Mackay-Steven, when he could reportedly have been signed this month for as little as £250,000, as quoted by the Daily Mail and other news outlets.
Is it shrewd business from Celtic to wait until the summer or a lack of ambition that they couldn't simply deal with Dundee United now?
That's the key question supporters were asking on Twitter after the signing, although there seems to be little doubt that the club are - even if only in the summer - getting themselves a very handy player.
Here is some of the best fan reaction to Celtic's latest signing, with Mackay-Steven officially no longer a Dundee United star as of this summer...
How miserable are Celtic these days? Wont even fork out 250,000 to get Mackay-Steven just now— Paul (@PaulMcCormick3) January 16, 2015
Celtic should've payed a bit of money and got Mackay Steven now, where's the ambition— Matt King (@kingmatt96) January 16, 2015
Like the look of Gary Mackay-Steven and think he'll be a great addition to our side. Also happy Celtic have a 5* skiller on FIFA ;)— Dean Craig (@Dheano) January 16, 2015
Well if that's Gary Mackay-Steven on his way here in the summer please let this be the last season we endure James 'Potential' Forrest— Brother Stannis. (@Stann1s) January 16, 2015
Think Mackay Steven is a good signing but we could be doing with him the now— Tom McEntee (@TomMcentee10) January 16, 2015