Olivia Smith’s brilliant solo goal shocked the visitors, who had dominated possession, before Fuka Nagano added a quickfire second for the hosts on the stroke of half-time.
United, who had won seven straight league games, struggled to recover and Liverpool came out after the break with more purpose, buoyed by the home crowd.
They were rewarded as Smith added a third goal from the penalty spot when WSL debutant Alejandra Bernabe was tripped in the box by Jayde Riviere.
United managed to grab a late consolation through captain Maya Le Tissier but it was too late to unsettle the hosts.
Victory continues a remarkable start to interim Liverpool manager Amber Whiteley’s tenure, just days after they booked their place in the FA Cup semi-finals with victory at Arsenal.
“I’m just so proud of the team. We prepared well this week and they went out there and executed the gameplan so, so well,” said Whiteley.
“We had to weather a bit of a storm in the first half. The two goals just before half-time gave us huge confidence.
“[The fans] have been waiting for that. We want to play football that they can get behind and be proud of. We did that tonight.”
Marc Skinner’s United side had only conceded six goals in the WSL prior to Friday’s defeat, which all-but ends their chances of catching runaway leaders Chelsea.
