Dutch striker Emnes raced clear onto Julio Arca's late throughball and held his nerve to slide the ball beyond Blades goalkeeper Steve Simonsen for only his third goal of the season, clinching a 2-1 win and extending Boro's unbeaten run to five matches.
His winner was cruel on United, who had cancelled out Matthew Bates' first-half opener for the visitors through Matthew Lowton before half-time.
Micky Adams' side were chasing the winner when Arca unlocked their defence, but slumped into bottom place as relegation rivals Preston secured a point at Portsmouth, while Scunthorpe upset leaders QPR.
Source: PA
Source: PA