Johnstone saved Reed's initial header but could not gather the ball on the greasy surface and the Sunderland skipper pounced to smuggle it into the net.
Undeterred, United levelled again with substitute Angelo Henriquez's incisive pass allowing Larnell Cole to nudge into Tunnicliffe's pass and the midfielder claimed his first goal of the season with a sweet finish.
Pickford made a superb save to deny Tunnicliffe a second after he latched onto a Henriquez through ball but the Wearsiders looked just as likely to grab a winner as Johnstone made smart saves to keep out a Mitchell free-kick and a miskick by Macheda from a corner.
United: Johnstone; Veseli, Fornasier, Thorpe, Vermijl; Bebe, Ekangamene, Cole, Tunnicliffe, Lawrence (Henriquez 60); Macheda. Subs not used: Sutherland, James, McCullough.