Watford manager Nigel Pearson made plenty of changes to his squad for the rescheduled affair. Mason Barrett, Callum Whelan, and Joao Pedro all made their first starts, and there was even a senior debut for Henry Wise - the son of former Chelsea captain Dennis - off the bench.
Tranmere, handled by the pleasingly named managerial team of Micky Mellon and assistant Michael Jackson, hosted the match on the much-maligned Prenton Park after the original replay was postponed on Jan. 14. Despite the best efforts of the pitch staff, the surface wore the scars of heavy usage from Tranmere, Liverpool Women, and Liverpool's youth sides, and was patched up with generous shovels of sand.
Emmanuel Monthe scored in the original 3-3 draw at Vicarage Road and opened the scoring during Thursday's scuffle with a deflected effort in the 36th minute.
Kaylen Hinds leveled for Watford midway through the second half following a goalmouth scramble before Mullin, who was brought on moments before the Hornets' equalizer, notched the winner in the 104th minute.
Watford were outmuscled and outfought by their modest opponents. They now begin a short break before returning to Premier League duty against Everton on Feb. 1.