Goals from Luke Rodgers and Craig Westcarr in either half put the Magpies in command, but it was the towering Dane's handiwork that maintained County's unbeaten record on home soil.
Schmeichel denied Joel Grant with a firm hand when one-on-one in the first half, then kept out Steven Schumacher's penalty on 76 minutes after John Thompson had pushed Grant.
That secured County's fifth clean sheet of the campaign and Johnson said: "Kasper was tremendous but that's what he's paid to do. He got a really firm hand on the first shot and the penalty save was sublime because it was low down to his right.
"He's a big player for us and saves of that calibre is one of the reasons we brought him to the club. That's our second consecutive clean sheet, so I'm delighted.
"When I was a player, clean sheets were like scoring goals for me, so to get another one and three points is important."