The home fans felt Serbia international Mitrovic should not have been on the pitch when he found the net having earlier avoided a second booking for a clumsy challenge on the keeper.
Mitrovic made the most of his luck and slotted home on 44 minutes to put Rafa Benitez's side one point ahead of second-placed Brighton & Hove Albion.
Chris Hughton's side had briefly replaced them at the top earlier in the day after Tomer Hemed scored twice in their 4-1 destruction of Burton Albion.
Below them, Huddersfield Town posted a fifth straight victory in all competitions with a 2-1 win at Queens Park Rangers to leapfrog Reading, who drew 0-0 with Barnsley.
Derby County, trailing 3-0 at the break to Bristol City, recorded the day's best comeback after a Darren Bent double and a goal from Tom Ince evened things up, while Cardiff City dented Leeds United's promotion hopes with a surprise 2-0 win at Elland Road.
Former European Cup winners Nottingham Forest and Aston Villa both lost, 5-1 to Norwich City and 1-0 to Ipswich Town respectively.
(Reporting by Neil Robinson; Editing by Toby Davis)
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