It looked like the Hoops would get their just rewards for their part in an unusually one-sided game between the Glasgow giants.
However, two minutes later, in a rare Rangers attack, McCulloch headed a Steven Davis corner past Artur Boruc to give the visitors an undeserved but welcome point.
Parkhead boss Tony Mowbray and the Celtic fans must be wondering how they failed to take a victory from this game.
The hosts dominated almost from first to last whistle, missing chances galore following the controversial decision by Conroy to disallow what looked like a perfectly good Fortune goal after 19 minutes.
Following a sliced shot by Aiden McGeady which spun high in to the air, the Celtic striker had the ball in the net after winning the challenge with McGregor on the goal-line.
However, to the anger of the home side and their supporters, Conroy, refereeing his first Old Firm game, pointed for a free-kick for the visitors.
From then on it was remarkably one-sided with Rangers offering little as an attacking force.
On the half-hour, with the Parkhead side in total control, midfielder Barry Robson volleyed high over from eight yards out after latching onto a Fortune knock-down.
Moments later, Rangers striker Kyle Lafferty was perhaps fortunate only to be booked for a late challenge on Celtic right-back Andreas Hinkel.
As the game became increasingly hostile, Light Blues skipper David Weir brought down Georgios Samaras at the edge of the box and would have been happy to see McGregor making a comfortable save from Danny Fox's free-kick.
Then, in the 36th minute, Robson headed a Hinkel cross off the top of the bar with McGregor beaten before McGeady missed from even closer range after another inviting cross from the Hoops defender.
Minutes from the break, with the visitors reeling, Samaras burst through the fragile Rangers defence only to side-foot his effort past the post before McGregor grabbed McGeady's cross at the second attempt to thwart another chance for the home side.
Davis was put into a supporting role behind Kris Boyd for the start of the second half.
But within two minutes of the restart the Hoops missed another half-chance when Samaras miscued his shot from the edge of the box as the visitors again failed to clear their lines.
Then, after Rangers skipper Weir lost possession when up for the Ibrox club's first corner of the game, Celtic broke with pace but the move ended when McGeady fired an angled shot wide of the target.
The theme of the first half continued.
In the 51st minute, Republic of Ireland midfielder McGeady missed another glaring chance, shooting high over from 12 yards after a Maurice Edu mistake had granted the home side easy possession.
Rangers had stemmed the tide a little but presented their hosts with an opportunity in the 67th minute defender when Sasa Papac fouled Robson 20 yards from goal.
However, the former Inverness and Dundee United player drove the resultant free-kick over the bar before Samaras, on the break, fired a weak drive at McGregor.
McDonald replaced midfielder Marc Crosas with 20 minutes remaining and in the 79th minute the substitute showed his team-mates the way to goal.
McGeady twisted and turned on the edge of the box before floating a cross in and the Australia international headed past McGregor from less than six yards out.
It looked all over for Rangers but two minutes later McCulloch dramatically headed Davis' corner from the right past Boruc and sparked relieved celebrations among the travelling support.
Moments later, as Celtic pressed again, McGregor produced a world-class save from Samaras, tipping the Greek striker's goalbound shot over the bar.
The game fizzled out and it was those in Light Blue who celebrated a significant result.