The Blues midfielder headed home the opening goal at Meadow Lane, with Jimmy Spencer and Alan Sheehan also on target for the Magpies. It was Mullins's second appearance for his new loan club.
The result also saw County climb out of the Sky Bet League 1 relegation zone into 19th spot.
Meanwhile Wade Elliott was sent on as a 32nd minute substitute for Bristol City in their 1-1 draw away at Oldham Athletic.
Elliott replaced his namesake Marvin Elliott in the League One fixture at Boundary Park after his team-mate sustained multiple injuries following a clash of heads.
Charlee Adams played the full 90 minutes for Lincoln City on Saturday in their Skrill Premier League away fixture at Hyde.
The Imps won the seven-goal thriller 4-3 thanks to a stoppage time winner.
Another Blues player in lower league action was defender Ryan Higgins with Leamington.
Higgins played the full match as the Brakes played out a 1-1 draw away at Gainsborough Trinity in their Skrill North fixture.
Further afield Darren Ambrose completed 86 minutes for Apollon Symrni as his side beat Xanthi 2-0 in their Greek Super League clash.
The three-point haul moved Apollon Symrni out of the relegation zone and up into 13th place in the league.