It earned City their second successive win as new manager Sven-Goran Eriksson maintained his 100 per cent record in the Premier League.
And what a difference Eriksson's arrival has made with a buzz of excitement and expectancy replacing last season's more sombre and subdued mood.
While Eriksson has spent £40million in pursuit of silverware, ironically it was the home produced Johnson who eclipsed many of City's more established players.
After an uninspiring opening, Johnson broke the deadlock in the 43rd minute with a move which he initiated and ultimately executed.
Johnson made a decisive burst through the centre of midfield before taking a return pass from Brazilian Elano and then curling an exquisite shot with the outside of his boot into the top corner from well outside the penalty area.
It was the only meaningful action of the dull opening period as Derby failed to create a clear-cut chance, having dropped striker Robert Earnshaw - their £3.5million record buy - and choosing to play a stifling 4-5-1 formation.
City remained the livelier in the second half but rarely threatened to increase their lead, although Elano and Stephen Ireland both had decent attempts at goal.
Derby showed plenty of enterprise but lacked firepower in the final third of the pitch until manager Billy Davies finally threw caution to the wind by bringing on Earnshaw for the final 20 minutes in a last-ditch bid for his side to claw their way back into the match.
But it was too late for Earnshaw to make any impact and it was City's substitutes, Geovanni and Valeri Bojinov, who were by far the more dangerous of the replacements.
Brazilian Geovanni made several dangerous raids down the right wing but nobody was able to get on the end of his dangerous crosses.
And in the dying minutes, jubilant fans started chanting the name of Eriksson who has quickly become a favourite.
City have every cause to feel confident as they will go into Sunday's local derby ahead of rivals Manchester United in the table - something of a rarity in recent years.