After going 1-0 up in the first half and being level at the break Selkirk were overrun by their Highland League opponents at Station Park.
After some early Nairn pressure, it was the visitors who broke the deadlock on 17 minutes. A loose ball dropped perfectly for Jack Hay to arrow a half-volley into the bottom right corner of the net.
County were back on level terms 11 minutes later when Conor Gethins rolled the ball to his left for the on-rushing Alan Pollock to crash into the net.
Selkirk had the first chance of the second half four minutes after the restart when a cross from the right was laid off by Frazer Neave to Garry O’Connor but the former Scotland striker’s side-footed effort was high and wide. A minute later, goal-scorer Hay drove in from the right and screwed a low effort across the Nairn goal which trickled just past the upright.
Two goals in as many minutes from Connor Gethins and Alan Pollock, swung the tie in County’s favour.
Jon McInally came close for Selkirk when his free kick on the edge of the penalty box bounced off the crossbar before County wrapped up the win when substitute Sam Urquhart knocked in a rebound and added his second of the game with three minutes to go.