Mannion was out of contract and although Blues offered her the opportunity to stay and sign a new deal, she declined and has opted to seek a new challenge.
The 23-year-old defender joined Blues from Aston Villa Ladies in August 2013, signed by David Parker ahead of the Club’s Champions League campaign.
Mannion, who had established herself as a key figure in the First Team squad and was voted Player of the Season by supporters at the Club awards evening in May, made 136 appearances in all competitions.
Sarah Westwood, Blues Women General Manager, said: “Ever since I joined the Club, Aoife has been a pleasure to work with and I have enjoyed getting to know her over the last nine months. She is an absolute professional and we all wish her every success with the next steps of career”.
The defender made her Blues debut in the Champions League during 1-0 home win over Finnish side PK-35 Vantaa as Parker’s side progressed to the last 16, before making six appearances in the competition.
While in the Women’s Super League she made 94 appearances, scoring on seven occasions.
Mannion played 19 games in the 2014 season where Blues narrowly missed out on the title, finishing in third and only one point behind winners Liverpool. She also made 20 appearances in the league during the 2018/19 campaign, helping Blues to their highest ever points total in WSL history.
In the FA Cup, she made 12 appearances including one at the Final against Manchester City at Wembley, in the Club’s first appearance at the national stadium.
The Club would like to thank Aoife for all her efforts and wish her every success in the rest of her career.
Blues have secured the talents of Claudia Walker, Connie Scofield and Emma Follis since the season's end, Rachel Williams has also signed a new contract, and the Club are in advanced negotiations to bring in new players, to be announced in due course.