The 22-year-old initially joined the Irons - and moved to England for the first time - in 2018 from BSC Young Boys.
This season, she has already made nine league appearances for the London side but has chosen to move to Merseyside.
The news comes after the club's official Twitter account posted a tweet on Friday teasing a new signing.
What she said
After signing for Everton, Lehmann spoke of her excitement to join the blues and develop under Willie Kirk
“It feels amazing to be here,” Lehmann told EvertonTV. “Everton is a big club and they have done so well this season. I’m so excited to play, to score goals and win with Everton.
“For me, it was my first choice to come here and I am so happy it has happened. I’m really excited to meet everyone, to be with the squad. I want to score goals and bring my strengths to the team.
“The manager also wants me to use my pace, to make runs, not just for me but also for my teammates. I hope Willie can bring me to the next level. I think I have potential and I can work on a lot of things.”
She is the second recent signing on Merseyside, with Jill Scott having joined the blues last week.
“I’ve watched their games and I know how well Everton have played this season,” she added.
“And we signed Jill Scott just a week ago, she is a really big player. There are big players here and that makes me even more excited because I can learn from them and they can teach me.
“Hopefully I can bring something extra to the team and keep improving.”
More to follow