Bianca Baptiste had put the hosts ahead in the 15th minute before Lucy Somes restored parity three minutes later.
In the 31st minute, Fern Whelan gave the visitors the lead for the first time in the duel.
Umotong, brought on in the 76th minute, then increased her side's lead ten minutes after before adding a second to seal another cup win.
The 23-year-old has now scored five goals in six games in all competitions for the Seagulls this season.
The win saw Brighton climb up to second of the Group 2 South standings with six points and they welcome Sheffield at Culver Road in their next league fixture on November 12.