The defender joins the Club on a two-year deal from Birmingham City and will wear the number two shirt.
The 23-year-old England Under-23 international is one of the most highly regarded young defenders in the domestic game and was voted into the 2019 PFA WSL team of the year.
Mannion graduated with a first-class honours degree from Birmingham University in 2017, combining her studies alongside her career at Birmingham.
Mannion said she was thrilled to be joining City and helping in the Club’s quest for continued success as well as looking to further her knowledge and development.
“I’m so happy to be here, so happy to have signed and I can’t wait to get started” Aoife told CityTV.
“The appeal of the club speaks for itself with the league titles and trophies the Club has won.
“The accolades are there and I just want to try and play a part in ensuring that continues to happen.
“When I spoke with Nick (Cushing) about making the move, we just seemed to be on the same wavelength and wanted the same things.
“We want to keep being successful and have a real pop at Europe which is the one trophy to have eluded us so far.”
Already an accomplished defender, Mannion says another huge incentive about making the move to City was the chance to learn and develop her game from playing alongside such experienced internationals as Steph Houghton and Demi Stokes.
“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,” Mannion added.
“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.
“As a defender I hope that I can help keep out the goals. The part of the game I enjoy the most is one-on-one defending but now I’d like to be known as a player who can help build from the back.
“I’m someone who likes to help support players on the ball and help build attacks form the back.
“Now I’m really looking forward to coming in and meeting all the players and I can’t wait to get started.”
Mannion also revealed she was hugely excited about the opportunity to sample playing in the Champions League once again after a brief experience in the tournament with Birmingham.
And she said she was determined to leave no stone unturned in her quest to try and help City achieve success in Europe.
“I’m so excited about playing in the Champions League,” Mannion added.
“I’ve only played in it once for Birmingham and that was six years ago and it wasn’t until the experience had gone by that you really appreciated it.
“So, I can’t wait to start and, more importantly, I can’t wait to get to the end of it as that would mean we’ve got into the final!”
Commenting on Mannion’s signing, City manager Nick Cushing said: ““I’m really excited about Aoife joining the team and I think she’s going to give us something we haven’t had over the past season.
“Aoife was someone we knew was a very high-quality player and we tracked over the last season and we knew that she would fit not only into what we want as a player but also as a person.”
“I think her development over the past three seasons at Birmingham has been incredible. She’s become a real dominant defender and such a good player in the Women’s Super League and she’s still very young and can become even better.
“She can play right-back, across the two central defensive positions so not only does she gives us different options but she comes in with a huge desire to play.
“So to add her to our squad for next season is a real asset for us and we want to play a big part in helping Aoife to become an even better player.”