Mooney netted four times in as many matches during the month as his side raced to the top of the league table.
The 28-year-old was thrilled to pick up the award, telling the club's official website: "It's fantastic for me and I'm delighted but it's as much an award for the rest of the team as it is for me."
Orient manager Russell Slade added: "I'm very pleased for David to get the award and the recognition he deserves. He's one of a number of players who has started well for us this season. His nine goals have really capped off some excellent performances."