Coventry City got themselves back to winning ways in League One after a stunning second half comeback courtesy of strikes from debutant loanees Chris Dagnall and Chris Maguire.
Both sides looked set to go into the interval on level terms until Ben Reeves was bundled over just inside the area, referee Jeremy Simpson then pointing to the spot and Williams converting just moments before the break.
The Sky Blues completely turned the game on it's head though when Dagnall 's looping shot beat Ian McLoughlin midway through the second half and Maguire rifled home two superb free kicks in the last five minutes to lift them back into the top half of the table.
City boss Steven Pressley made three changes to the side that lost 3-0 at home to Rotherham on Tuesday nighht, handing debuts to deadline day loan signings Chris Dagnall and Dan Seaborne and bringing back full-back Cyrus Christie at the expense of Aaron Phillips, Adny Webster and the injured Billy Daniels.
With 7,000 travelling City fans in attendance the stage was set and the expectancy of excitement very nearly came early on despite it going the way of the home side.
The first chance of the afternoon fell to MK Dons when Ben Reeves slid a through ball for Daniel Powell who went one-on-one with City 'keeper Joe Murphy only to be denied by a leg save just inside the area after six minutes.
For all of the energy and industry of the first half though that was the only real chance of the first half to speak of until the 40th minute when Callum Wilson came inches away from giving City the lead.
A bouncing ball in the Dons area was chested down by Dagnall into the path of his strike partner who fired first time but saw the slightest of deflections steer the ball just around the foot of the far post.
City could feel aggrieved at falling behind though just a minute before the break when Reeves was felled just inside the area as the Sky Blues tried to clear their lines and the home side were awarded a penalty.
Williams then stepped up to convert the penalty, sending Joe Murphy the wrong way and handing Dons a slender lead.
There was no doubt that City got off to the better start after the break though.
John Fleck kicked things off with a long range effort after 49 minutes which Ian McLoughlin had to turn around his post.
The shot sparked a mini revival for the Sky Blues as they had two scrappy half chances almost immediately after, on both occasions Callum Wilson almost getting the killer touch just six yards out although frantic defending kept the Sky Blues at bay.
The early surge definitely gave City the impetus and Carl Baker went close three minutes before the hour when he received a quick throw, cut inside his man and pulled his shot from inside the are just wide of the near post.
They then got the rewards their efforts deserved, albeit with a touch of fortune, when the ball fell to Dagnall 20 yards and his half volley took a deflection to loop the ball over McLoughlin and into the net.
Dons nearly reinstated their lead just seconds later when a troublesome low cross from Powell found its way to the feet of Dale Jennings whose first touch just four yards out saw the ball flash across goal and just wide.
Debutant loanee Chris Maguire then came just inches from making a statement of his own just six minutes after coming off the bench when he was handed a dead ball opportunity just on the edge of the area on 69 minutes, his strike rising just over the bar from the edge of the area.
McLoughlin had to be on his toes with ten minutes to go though. A thrashed in low cross from Cyrus Christie found it's way onto the stretched out toe of Wilson late on with the Dons 'keeper having to get down quickly at short notice to make a parried save.
He was called to action again two minutes later when Christie played in Maguire to race into the area and fire from a tight angle, McLoughlin standing up to beat the ball away.
He could do nothing about City's second though with four minutes to go when another Sky Blues loanee got on the scoresheet.
Maguire, having gone close from a set piece earlier, stepped up with a 20-yard free kick opportunity, lifting it over and around the wall and beyond McLoughlin's reach to see it creep inside the post and send City's 7,000 into delerium.
Just when City fans did not think it could get any better though it did as the game drew to a close. And it was Maguire again with another stunning free kick which put his first in the shadows.
Further in distance and more acute in angle, the Scottish international rifled a curling dead ball into the top corner to wrap up three points for City and lift them back into the top half of the League One table.
Coventry City: Murphy, Christie, Seasborne, J Clarke, Adams, Baker (c) (Maguire 63), Fleck, Thomas, Moussa (Phillips 89), Dagnall (Webster 90), Wilson
Subs: Burge (GK), Haynes, Garner, Maund
MK Dons: McLoughlin, Lewington (c), Hodson, Williams, Gleeson, Potter, Reeves (Carruthers 88), Bamford, Powell (Smith 75), Spense, Jennings (Alli 71)
Subs: Burns (GK), Flanagan, Baldock, Banton