The 23-year-old defender arrives from Birmingham City and has penned a two-year deal.
Regarded as one of the most highly rated youngsters in the FA WSL, she was named in the 2019 team of the year and has opted to take the number two shirt.
WATCH | Mannion's first interview
Mannion earned a first-class honours degree from Birmingham University whilst at her former club and having committed her future to City, she is keen to learn all she can from her new team-mates.
“Hopefully I can also bring some of my personality out on the pitch but there’s a lot that I’ve still got to learn,” she said.
“When I come in for pre-season I know it’s going to be a massive learning curve but that’s the part of joining that I’m most excited about.
“Getting to know all the girls, getting to train with them and then playing games and hopefully helping us to win trophies going forward.
“There are so many fantastic players here that have made such a career for themselves and inspired so many girls. Hopefully, I’ll be able to learn from them too.”