Stevens but United then responded with more pressure of their own.
Brady shot too high after good play by Jesse Lingard and Will Keane and Lingard then saw a shot deflect onto the top of the net following a Tunnicliffe lay-off. Tunnicliffe and King had further efforts while Josh Webb got his finish all wrong after being presented with a decent opportunity on the right-hand side of the area.
Scott Wootton, who had headed past his own post with a clearance at the start of the second half, could not keep his shot down after being picked out by Lingard as penalties loomed. But there was a stirring finale with Johnstone scrambling away a Gardner header and then blocking another shot from the midfielder. In the dying seconds, Lingard did well to pull a Brady pass out of the air but saw his powerful drive beaten away by Siegrist's best save of the night.
Then came the drama which ended in United's favour - thanks largely to Johnstone's heroics, which gave the supporters something to cheer and means it is now full steam ahead to the Manchester Senior Cup final at Manchester City and the chance of more silverware next Thursday.
United: Johnstone; Fornasier (Vermijl 46), M.Keane, Wootton, Fryers; Cole, Thorpe (Brady 50), Tunnicliffe, Petrucci (King 71), Lingard; W.Keane. Subs not used: Coll, Giverin.