The 24-year-old, who scored in Derby’s recent League Cup win over Manchester United, came up with the winner in the 77th minute to leave Chris Wilder’s side frustrated.
The Beverley-born striker latched on to Craig Forsyth’s wicked cross from the left to steer in from close range after United had battled back from conceding after just 19 seconds.
It was only the Blades’ second defeat in a dozen games but they missed the chance to reclaim top spot, Middlesbrough the new leaders, courtesy of a superior goal difference, after last night’s win at Sheffield Wednesday.
Derby, who started the campaign at apace, are up to fifth with only their second win in eight games.
United, who host Stoke City on Tuesday, had controlled the majority of the first half but conceding the crucial goal had been looking more likely in the second period as Frank Lampard’s side began to force more pressure.
Dean Henderson made a quality save from Tom Lawrence and then Martyn Waghorn curled one effort just over the crossbar before the hosts struck.
Yet the Blades had overcame that horrendous start - Derby’s Craig Bryson scored in the opening minute - to quickly build their way into the game.
They had not even touched the ball before Bryson calmly slotted home after a slick home passing move saw them dissected.
However, Wilder’s side did not allow that setback to unsettle them and soon started creating problems of their own with the stylish football that had seen them emerge as early Championship leaders.
With John Fleck and Oliver Norwood dictating from the middle, and Chris Basham popping up all over, they caused Derby plenty of issues.
The problem was, for all they created promising opportunities, their final ball was too often wayward, Enda Stevens and Mark Duffy both guilty in such instances.
Furthermore, although Derby were starved of any clear-cut chances themselves, they always remained threatening on the counter-attack.
Indeed, United’s record-signing John Egan was called upon to make a timely intervention after Basham had been pickpocketed by the rapid Harry Wilson, the on-loan Liverpool youngster who scored a wonder goal for Wales in midweek.
They did, however, draw level when Basham appeared unmarked at the far post to finish off some fine trickery inside the box from Fleck in the 41st minute.
It was the least they deserved but Derby reminded them of their threat in the dying stages of the first period.
Waghorn put a free header wide and then Marriott’s volley forced a fine save from Henderson before Mason Mount pulled a shot just wide.
Clearly, the warning signs were there, then, so United should not have been surprised by what entailed after the break.
Derby County: Carson; Bogle, Tomori, Keogh, Forsyth; Huddlestone; Mount, Bryson (Lawrence 36), Wilson; Waghorn (Jozefzoon 71), Marriott (Davies 86). Unused: Roos, Johnson, Nugent, Malone.
Sheffield United: Henderson; Basham (Washington 80), Egan, O’Connell; Freeman, Norwood, Fleck, Stevens; Duffy (Coutts 65); McGoldrick, Sharp (Clarke 65). Unused: Moore, Stearman, Johnson, Craine.
Referee: Oliver Langford