The 18-year-old has already made four substitute appearances for the first team this season, impressing manager Sam Rose enough to secure her immediate future with the club.
Richardson has been with Oxford United since the age of seven and played a key role in the development squad’s treble-winning season last year.
She said: “It feels great. It’s something I’ve wanted for a while, so to finally get it done feels good.
“It’s a massive family club that’s always willing to develop you and work with you and so it’s always been a place that I’ve wanted to be.
“I’ve never felt the need to move. The jump from the devs to the first team has been really challenging, which is what I wanted.
“It’s a lot more demanding physically than development football, it’s faster and stronger, but that’s exactly what I wanted.”
Rose said he was “delighted” to see Richardson commit her future to the club.
He added: “She’s a talented player who has shown us since stepping up to the senior environment that she has plenty of potential, and she has established herself as an important member of the squad moving forward.
“Ellie is part of a strong midfield unit and her continuing to play with and compete against experienced players like our captain Leah Burridge, Jenna Legg, Kate Wiseman and Vita Budaieva will only help her continue to progress.
“This marks a moment to celebrate the work that our dedicated staff do at youth level, supporting Ellie-Mae to this point.
“We thank them for their continued hard work in developing the next generation of local players and we hope Ellie’s journey can be an inspiration to them and our pathway.”