Grainger, whose previous contract was due to expire this summer, has put pen-to-paper on a new two-year deal, commiting himself to the Club until 2019.
The 20-year-old will now battle with fellow goalkeeper Sam Sargeant for the number one spot ahead of the O’s National League campaign.
Speaking on committing his future to the Club, Grainger told leytonorient.com; “I couldn’t be any happier, as this is something I have been wanting to get sorted all summer. I want to be here and love the Club.
“I have been here almost nine years now, and want to do everything I can to try and put things right from last season.
“It will be a really good battle with Sarge for the number one spot this season, and something that I am very much looking forward to.
“I’ve been working hard during the off-season and can’t wait to get going on Monday. It is an exciting time for the Club and I am delighted to be a part of it.”