The talented youngster, who gave a 70-minute cameo performance, was in outstanding form on his return to first-team duty and marked Villa's easy success and his own quality display with a brilliant goal from 25 yards.
Villa completely dominated the game, sealing their success when they ran riot in the first half, with two Conor Hourihane goals and one each from Tammy Abraham and Grealish.
The belated return of the latter, which was marked with the talented youngster being named as Villa's captain, produced an immediate improvement as demonstrated by their first-half onslaught.
Villa had won just two of their 13 games without Grealish but completed a well-deserved double over the Rams, who slipped to their third successive defeat.
Derby, showing six changes from the side beaten in midweek by Nottingham Forest, were under early pressure and there was an ominous warning of what lay in store. Only extremely poor finishing
by Hourihane saved them from falling behind in the opening five minutes.
The Irishman, however, made amends in the ninth minute. Goalkeeper Scott Carson was left stranded in the middle of the penalty area when Abraham was able to nudge the ball back into the middle to the unmarked Hourihane, who had the simplest of tasks to turn the ball into an empty net.
Carson continued to have an anxious time behind a Derby defence which was opened up far too easily, and Villa should have increased their lead in the 18th minute when Albert Adomah casually lofted his shot over the bar with the opposition defence badly wrong-footed.
As it was Derby struggled to find their promotion-chasing form. Eventually Duane Holmes began to make an impact as he set up several promising attacks to expose Villa's uncertain defence.
But in the 37th minute Derby's rearguard was again stretched and a long, low crossfield pass was turned into the net at the far post by the sliding Abraham.
With Derby's defence being torn to shreds, a Grealish-led attack saw Villa notch their third goal in the 44th minute. Abraham again unselfishly set up Hourihane to score his second goal from close range.
The icing on the cake came in the second minute of added time. Glenn Whelan's corner from the left was delivered directly to the edge of the penalty area, where Grealish met it with a ferocious right-footed volley into the far top corner.
Villa calmly closed out the win in the second half, though it was tarnished by the loss of defender Tommy Elphick on a stretcher.
Grealish left soon afterwards to a standing ovation as Villa fans enjoyed an almost perfect afternoon.