Henderson, who has also had loan spells at Wycombe and York, has made six appearances for Kevin Dillon's side, keeping two clean sheets.
He will now stay at the Recreation Ground until Thursday, 1 April.
Henderson told BBC Surrey: "I'm delighted. Another month, another six games I think, so I just need to kick on now.
"I feel I've got a long way to go but I'm getting better and better each game."
The Irish stopper has replaced Mikhael Jaimez Ruiz in the starting line-up for the Shots.
Henderson said: "I get along great with Mikhael and it's unfortunate that he's lost his place but that's football. I need to play games and that's what I'm here to do."