Fotheringham, who signed from Ross County last summer, smashed home the winner after Enoch Showunmi had cancelled out Conor Coady's opener for the Blades.
Liverpool loanee Coady had taken less than two minutes to break the deadlock, rifling home from the edge of the box following a neat one-two with Chris Porter and he almost doubled the Blades' lead three minutes later but side-footed wide.
County found their feet though and were level after 26 minutes. The dangerous Jamal Campbell-Ryce fizzed a cross into the box which hit Showunmi on the chest and bounced in.
Showunmi was almost the villain soon after, but Sheffield United appeals for a handball by the striker in the County box fell on deaf ears.
The high tempo continued into the second half with both teams wasting chances before Fotheringham edged the Magpies ahead.
The Scottish midfielder headed Alan Sheehan's cross straight at goalkeeper George Long, but reacted quickest to the rebound and fired the loose ball into the roof of the net.