The young Irish striker was on target in the second minute of injury time when he latched on to Paul Hanlon’s headed assist to find the bottom left-hand corner of the net from 12 yards.
Although the goal prevented Terry Butcher from losing his first match in charge at home, it did little to provide any Christmas cheer for the Edinburgh club’s long-suffering fans.
Hibs have won just once at home in eight league games this season, while their confidence in front of goal remains at a low ebb.
They have now gone 10 matches in all competitions without scoring more than once in any game.
And it could have been much worse for the hosts after Thistle broke the deadlock four minutes into the second half thanks to an excellent individual goal by Kris Doolan who ended his run by lifting the ball over the advancing Williams.
But Thistle, arriving in the Scottish capital on the back of five straight defeats, failed to kill the game off and were left to rue a 75th minute miss by Ross Forbes who narrowly failed from 10 yards.
During a goalless first half, Hibs enjoyed most of the possession and created the majority of the chances.
Jordan Forster, Daniel Handling and Paul Hanlon all headed off target from good positions while Ryan McGivern had a left-foot shot saved by visiting keeper Scott Fox.
At the other end Kallum Higginbotham and Steven Lawless had efforts saved by Ben Williams.
As Hibs tried desperately to get back on level terms, Collins did his best to force an equaliser but failed to convert a close-range header 18 minutes from time as he nodded the ball straight into Fox’s arms.
However, the same player did manage to spare his new manager’s blushes late in the game to leave both sides disappointed with the outcome.