Second half goals from Ruel Sotiriou and Harry Smith were enough to give Orient the three points in front of a raucous home crowd at Brisbane Road.
The Bantams, who put in comfortably their worst performance of the season thus far, can take some comfort from the fact that despite defeat they still remain in the automatic promotion spots.
Story of the match
Despite both teams coming into the game ranking among the top scoring in sides League Two thus far, neither managed to create any clear cut chances in the early stages.
The first major opportunity of the match did not come until the 39th minute when when Bradford Captain Niall Canavan put a thumping header goalward after meeting a Callum Cooke corner but could only watch as his effort was superbly cleared off the line by Hector Kyprianou.
The game, however, started to open up as the second half began with Theo Archibald coming close to breaking the deadlock for the hosts just two minutes after the restart when the forward managed to cut in from wide and only saw his stinging low effort denied by a good save from keeper' Richard O'Donnell.
Kenny Jacket's side continued to mount the pressure and came even closer after 64 minutes when Smith got on the end of a perfectly weighted cross from former Bradford left-back Conner Wood and was only denied his second goal of the season by the underside of the cross-bar.
The O's would not have to wait to much longer for their opening goal though, Sotiriou putting them ahead with an impressive finish from a difficult angle after the ball fell to him at the back post in the 66th minute.
The hosts then manged to prevent any chance of late drama, bagging a second goal with eight minutes to go when Smith carried the ball from deep before expertly finishing into the bottom corner with a powerful strike from the edge of the box.
Bradford struggled to break down the Orient defence all day and can have few complaints over the outcome of the game - their first defeat of the campaign sees them fall once place on the table into third.
Leyton Orient on the other hand can take away a whole host of positives from the afternoon and now climb to sixth in League Two.
Man of the match - Harry smith
Summer signing Smith undoubtedly proved his worth to the Orient faithful with a brilliant performance in what could have been a difficult game for the London side.
The 26-year-old used his height and strength to cause problems for the Bradford defence all afternoon and never allowed them to settle.
Smith then capped off his influential display with a superb goal to seal the game for his side.