Leah Galton's 29th-minute goal gave her team something to build on but it wasn't until an hour later that Ella Toone wrapped up the points by lobbing the hosts' goalkeeper for an assured finish.
The Reds, who remain top of the WSL for at least another week, have won every game on the road in this league campaign so far, following the previous victories at West Ham, Tottenham and Villa's local rivals Birmingham City.
It was a strong start to the game by United, who looked to open the scoring in the first minute as Lauren James drove towards the defence with the ball close to her feet, but her right-footed shot from outside the area narrowly went wide.