Porter struck from the spot with three minutes remaining after Freddie Sears had fouled Ben Davies in the area.
The Blades looked more dangerous in a lacklustre first half.
But the closest they came to scoring was when John Brayford's far-post header from a corner was saved low down to his right by goalkeeper Sam Walker.
Tom Eastman's diving header from Brian Wilson's corner floated wide of the near post for the U's, before the visitors went close three minutes before the break when Davies' header flashed wide from Jamie Murphy's delivery.
Jose Baxter's free-kick sailed over the crossbar just before the hour mark for the visitors and the former Everton youngster came even closer to scoring soon afterwards when his fine curling effort came back off the foot of the post.
U's keeper Walker made a wonderful reflex save to deny Davies from point-blank range from Brayford's cross, before Porter struck the winner.