The O's are the only side in League Two to have won all three games this season thanks to Matt Lockwood's first half penalty.
Martin Ling's side have now made their best start since 1912.
Lockwood made no mistake from the spot in the 40th minute after Darlington defender Shelton Martis was penalised for a clumsy challenge on O's winger Shane Tudor.
The O's win was well deserved against the Darlington side, which felt the rath of home fans following a below par display.
However, the O's were still made to work hard for their victory with the home side guilty of several glaring second-half misses.
After wins over Macclesfield and Bury, Orient arrived in the north east with no shortage of confidence, and it showed in the opening exchanges with Tudor and Steele both testing Sam Russell with long-range efforts.
Orient then had Garner to thank for saving bravely at the feat of Simon Johnson, before the keeper almost turned Carlos Logan's low driven cross into his own goal.
Lockwood then gave Orient the lead from the penalty spot after Martis' challenge.
Wainwright wasted a good opportunity to draw Darlington level when he shot over the bar after latching on to Matthew Clarke flick on.
Martis then missed a chance to make up for his earlier error, hitting over Matty Appleby's cross.
But, it was a brilliant save by Garner that denied Darlington there best chance to equalise when the O's stopper pushed Johnson's header away.
Substitute Akpo Sodje was guilty of two glaring misses in the 80th minute. Akpo Sodje hesitated from Logan's cross, allowing Orient to clear and, with full time approaching Sodje was then denied another good goalscoring opportunity when his shot was blocked by John Mackie.