The Irishman was linked with the Magpies during last season, when Gudjon Thordarson was in charge, but it is current boss Steve Thompson who has finally captured is man.
McMahon, 24, is said to be an attacking midfielder, and he has made nine appearances for League 1 side Orient this season. In total, he has made 81 career appearances, scoring six times.
He wasn't involved in Notts' 2-1 FA Cup first round defeat at the O's, and instead he has been restricted to Reserve team appearances so far this campaign.
Should his switch be a successful one, Thompson may look to make it permanent after December 16, when the initial loan period is up.
McMahon told the Official Site, “I'm delighted to be here, and I know firstly I will have to fight for my place because the players have been doing really well here.”
He added, “The plan is for me to be here for a month with a view to a permanent move, but it is down to the manager and how I perform.”
NCM will have more reaction to follow.