The Stags would have leapfrogged leaders Lincoln City on goal difference after Danny Cowley’s charges were held to a draw against Northampton.
David Flitcroft’s men slipped to their first defeat against Newport in six outings as the club’s five-game unbeaten streak also came to an end.
Robbie Willmott bagged the only goal of the game in a scrappy affair at Rodney Parade which saw Stags an away game for only the second time this season.
Manager David Flitcroft named the same side which saw off Macclesfield last weekend.
The ground observed a minute’s silence in memory of Emiliano Sala and pilot David Ibbotson before the game.
The Stags hit the ground running as Nicky Ajose squandered the visitor’s best chance of the first half by firing over in the first minute.
Down the other end Joss Labadie lifted over the bar from close range.
But Flitcroft’s charges found themselves behind on the quarter of an hour mark.
The ball fell kindly for Willmott in the box, with the 28-year-old blasting his second goal in as many games into the roof of Jordan Smith’s net.
It was seemingly two 10 minutes later, only for Labadie to be ruled offside after an easy tap-in.
Jacob Mellis lashed marginally post the post from long range as the frustrated Stags tried to create something, making two changes at the break.
There was a seven minute delay early in the second half after a nasty clash of heads for Danny Rose.
A positive for Flitcroft is that Rose was able to walk off the pitch, despite a stretcher being called for.
Things just didn’t fall for the Stags on the day.
Tyler Walker couldn’t quite get on the end of CJ Hamilton’s cross at the front post.
Jacob Mellis and Willem Tomlinson both had strikes deflected for corners as the game opened up with 20 minutes left.
Smith kept the Stags in the game with a fantastic low save from Jamille Matt’s drive.
Ryan Sweeney headed just wide after being sent forward as a target man at the death as Stags missed out on top spot, though remain in second place.
Newport: Day, Demetriou, Poole, O’Brien, Willmott (Dolan, 88), Bakinson, Bennett, Labadie, Butler, Matt, Amond. Unused subs: Townsend, Hornby-Forbes, Kennedy, Crofts, Sheehan, Marsh-Brown.
Mansfield: Smith, Preston, Pearce, Sweeney, Hamilton, Mellis, Bishop, Grant, Benning (Jones, 45), Walker, Ajose (Rose, 45, Tomlinson, 59). Unused subs: Logan, Smith, Khan, Atkinson.
Attendance: 3,133 (362 away).
Referee: Graham Salisbury.