Versatile Scotland international Mitchell arrived at Arsenal in July 2013, following spells north of the border with Glasgow City and overseas at German side SGS Essen.
Swapping the Frauen-Bundesliga for the WSL, Mitchell played in an FA Women’s Cup Final win over Everton in 2014 and lifted WSL Cups either side of that Wembley win. She was back at Wembley to beat Chelsea in 2016.
Strong, athletic and good on the ball, the left-footer remained a regular with Arsenal before spending a spell last season on loan at north London rivals, Spurs.
On the international scene, Kirkcaldy-born Mitchell captained the Under-17s and Under-19s before receiving more than 60 caps for her native Scotland.
Now she will look to cement a regular spot in Kelly Chambers’ side.
"Emma is a great addition to the squad and is someone who knows what it takes to win and be successful," Chambers said.
"Personality and character is something i look for in players and Emma’s will definitely add to and fit with the group I have already.
"I am extremely excited to work with Emma, not only because of her ability but she is a very honest, hard working player who will add that competitive edge to everything we do."