Shane Tudor struck the all important goal with just 11 minutes on the clock to give his side their seventh home win of the season, although the visitors were ruing a number of squandered first half chances. That was to prove costly.
Tudor, who had missed the last two matches through injury, soon proved his value winning a corner in his side's first attacking movement of the match and just moments before proving match winner with his wonder goal.
Collecting a pass from Gary Alexander he cut inside a bemused Ryan Valentine and curled a rising left-footed shot past the despairing keeper Sam Russell from 20 yards.
However the home side were unable to build on that perfect start and were forced to rely on some resolute defending that saw John Mackie and his central defensive partner Gaby Zakauni in outstanding form.
The visitors' wastefulness in front of goal was also crucial to Orient's cause with Andy Cooke and Guy Bates squandering good chances to press home the midfield superiority enjoyed by the North-east side throughout the first half.
In contrast Orient were dependant on the lively Tudor to create chances before Paul Connor and Alexander, who might have done better with improved service.
The home side had a golden opportunity on the stroke of half-time to put the issue beyond doubt when Connor was felled in the box, but Russell produced a magnificent save to deny Matthew Lockwood from the penalty spot.
Orient increased the tempo after the interval, although a difficult pitch hampered their preferred passing game, but they did manage to pin Darlington into their own half for long periods.
Connor was only denied his first home goal since moving from Swansea a month ago when the visitors' man-of-the-match Russell produced a splendid finger-tip save and the hard working keeper then denied Mackie at the expense of the corner.
Orient continued to press forward in search of the second goal but were denied by a string of fine saves from Russell, although in contrast his opposite number Glynn Garner was rarely troubled.
As Darlington's frustration grew the match turned feisty with referee John Moss producing three yellow cards in the space of 11 minutes, two for Matty Appleby who found himself dismissed on the stroke of full-time.