Tamika Mkandawire put the hosts in front on 82 minutes with a header which came against the run of the play. He rose to meet Nicky Adams' cross, for whom this was the last game of his loan spell.
Mkandawire's sixth goal of the season plunged a dagger into the heart of Leeds and set Orient spirits soaring inside Brisbane Road.
It came after the O's had stayed solid in the face White Wave after White Wave for so long. But then, on Valentine's Day weekend, came the heartbreak.
Four and a half minutes in the stoppage-time, Charlie Daniels sliced the ball past keeper Jamie Jones as he attempted to clear a cross. It was almost the last kick of the game, and was the least that Leeds deserved for their efforts.
That the goal from such an unlikely source summed up how unpredictable this fixture has become in recent times.
It seemed like a kick in the teeth for Orient, but Geraint Williams' men can be well pleased with their battling performance and the point earned from it.
Leeds were wasteful of their possession and the chances they made throughout, and looked like they missed the injured Jermaine Beckford.
But Simon Grayson's team still played Orient off the park during a dominant first half display, when they ruled the game from the midfield.
Daniels had a tough time trying to contain Richard Snodgrass on the left, and Orient keeper Jamie Jones showed his class against attacks which came from all angles.
He produced two superb stops to keep the scoreline level when he turned Gary McSheffrey's point-blank header round the post on 25 minutes.
Then he got a hand to Leigh Bromby's piledriver of a half-volley on the stroke of half-time. He could not know then that the linesman's flag was up.
Scott McGleish and Ryan Jarvis spent more time tracking back than on their striker duties.
It was all hands to the pumps at times for the host's as they fought against a slick show of passing by the Yorkshire visitors.
It all meant that Orient managed only two shots on target by the time they trudged down the tunnel for the break.
Jason Demetriou firing into Ankergren's arms during an industrious first-half display, and Thornton warming the keeper's gloves were the only time the Leeds stopper was troubled.
The second period began with Orient on the back-foot again. Daniels got off to a good start dispossessing Snodgrass, but the Scot got clattered by Chambers on 50 minutes, and captain Richard Naylor just fired over from the resulting free-kick.
Goalscorer Mkandawire impressed at the other end of the pitch with centre-back partner Ben Chorley.
He had his own hands full with Becchio, but rose to the challenge. A highlight came when he stole the ball from under Becchio's nose on the six-yard line when Snodgrass played in another dangerous cross, on 75 minutes.
Soon afterwards, Orient pulled out in front. Jason Demetriou's corner was lumped clear to Jimmy Smith, who first-time passed the ball out to Adams on the left wing.
The winger ensured he gave Orient a present to remember him by, with his pin-point cross to the far edge of the 12 yard box.
There Mkandawire was lurking who rose and sent the volume made by the home fans sky-high.
The Leeds fans were cheering loudest when their team escaped with a point, and there was a pitch invasion too, but for Orient this was an encouraging performance ahead of next week's journey to Tranmere Rovers.
LO: Jones, Purches, Chorley, Mkandawire, Chambers, McGleishm (Téhoué 53), Thornton (Smith 62), Demetriou, Daniels, Jarvis, Adams.
Subs: Morris (GK), Tehoue, Cave-Brown, Baker, Briggs, Smith, Scowcroft.
Attendance: 8,013, (2,891 Leeds fans)