Two goals from stand-in skipper Gary Alexander saw Orient stretch their unbeaten home run to seven as they dealt Mansfield's promotion hopes a bitter blow.
Alexander struck after just six minutes when he prodded home his eighth league goal of the season after Mark Peters had knocked on Justin Miller's long throw.
But after that dream start it was Mansfield who took a grip on the proceedings and it came as no surprise when they levelled on 33 minutes following their sixth corner, when Jake Buxton headed in from 12 yards.
Orient regained their composure in the second half in what manager Martin Ling described as the best 45 minutes since taking charge.
Orient regained their lead 20 minutes from time when David Hunt's corner was only cleared as far as Tom Newey and his effort from 25 yards took a wicked deflection and flew into the corner of the net.
Orient warpped things up ten minutes from time when Jabo Ibehre's wriggling run left two Mansfield defenders floundering and set up Newey to provide the perfect cross for Alexander to fire his ninth of the season.
After the game, Orient manager Ling said: "We were given a good start with the early goal but we failed to build on it and took a step backwards.
"In the first half I think Mansfield were a little bit better than us." Indeed, after Buxton's leveller O's keeper Glen Morris made two excellent saves to deny the ever dangerous Junior Mendes, while all Orient could offer before the break was a wild volley from Miller that flew over the bar.
But the second half was a different story and apart from the two goals Alexander almost hit the target again when he latched onto Hunt's astute through ball but fired just wide.
Disappointed Mansfield boss Keith Curle said afterwards: "It was one of those times when I didn't need to say anything. You could feel the disappointment in the changing rooms after the match."