The Reds triumphed 2-0 against the Royals on the league’s opening night to ensure the perfect start to the new campaign.
Mannion, formerly of Manchester City, Birmingham City and Aston Villa, made her competitive debut for United and played the full 90 minutes, helping her new team register a clean sheet and claim the victory.
After the final whistle, the 25-year-old went to the pitchside interviewing area to speak about her first game for the club and the feeling among the group following the result.
“We’re absolutely buzzing,” the defender told MUTV.
“If someone would have said before the game that we’d have got two goals and a clean sheet we’d have taken that.