Early efforts from Lucia Garcia and Rachel Williams put Marc Skinner's team on the path to Wembley, before former Red Lauren James halved the deficit on the stroke of half-time.
But despite Emma Hayes's side's best efforts, United's defence – and Earps, in particular – performed brilliantly to thwart the reigning champions and see the Reds into the FA Cup final for the second season in a row.
"[It feels] so good," said Mary in an interview with BBC Sport after the match.
"This season has been so hard. It's been mentally brutal but I’m so glad, [we are] back for the final.