Will Keane's first half penalty and John Cofie's first Reserves goal gave United a third successive Manchester Senior Cup victory, and put the Reds firmly in control of the competition's group stage.
The only negative aspect to the evening was provided by an ankle injury to Anderson. The Brazilian midfielder landed awkwardly after taking a shot early in the second half, and left the field. Positively, he was soon back in the United dugout rather than seeking treatment.
From his new vantage point, the Brazilian witnessed a second half continuation of the first period's trends; namely Magnus Eikrem pulling the strings for United and County goalkeeper Owain Fon Williams repelling almost everything thrown at him by the hosts.
The visitors had the ball in the net inside two minutes, but Jake Simpson's stabbed finish was correctly ruled out for the striker's foul on Scott Wootton in the build-up. Keane responded with United's first meaningful effort, but his speculative volley drifted comfortably wide.
Fon Williams was soon called into action, as Ritchie De Laet's curling shot forced a spectacular parry, and the County goalkeeper was regularly involved thereafter. He was given a let-off in the 15th minute, however, when Keane fired wastefully over after Corry Evans' shot had been impressively blocked.
At the opposite end Johnstone, making his first competitive Reserves start, showed quick reflexes to turn away Jordan Rose's close-range volley, but Fon Williams was soon back in the spotlight with a sharp low stop from Nicky Ajose.
As United's pressure grew, Stockport's defending became increasingly desperate; a trend characterised by Reiss Lee's rash challenge on De Laet as the Belgian sought to pull back for a colleague. Keane assumed the penalty-taking duties, and clinically fired his effort inside Fon Williams' post.
Two minutes later, Evans appealed for another spot-kick, but was left disappointed as Andy Halls' shoulder barge escaped