The Villans were beaten 1-0 by Fulham in this showpiece fixture 12 months ago, but they came out on top this afternoon to break the hearts of their Midlands rivals.
Anwar El Ghazi and John McGinn scored either side of half time, with the latter given a huge helping hand by County keeper Kelle Roos, and Villa saw the job through despite substitute Jack Marriott's goal in the final 10 minutes.
Derby bombarded their opponents' goal but there was to be no way through, meaning the return of Premier League football at Villa Park next season for the first time since 2016.
Villa won both of the regular league fixtures between these sides this season by an aggregate 7-0 scoreline and they settled the quicker here.
Grealish had a good opportunity two minutes later following a poor clearance from Roos, but he sent his swerving shot over the crossbar from the edge of the box after cutting inside.
The breakthrough moment arrived just before half time as Ahmed Elmohamady sent a delightful cross to the edge of the six-yard box and El Ghazi got in front of Jayden Bogle to steer his header into the far corner.
Derby famously came back to beat Leeds United in the semi-finals, but their task was made twice as difficult with an hour played as McGinn nipped in to score a second for the Villans.
El Ghazi's shot deflected into the air, Roos failed to gather six yards out and the ball hit the head of McGinn and trickled over the line.
Lampard took action by making three subs in quick succession and that gave Derby an attacking threat that they had lacked for most of the afternoon.
Bogle blasted into the side-netting after getting away from El Ghazi when he had substitute Martyn Waghorn alongside him, while fellow sub Marriott - the hero at Elland Road two weeks ago - guided a volley inches wide on the turn.
Marriott once again made a big impact from the bench, though, as he controlled Florian Jozefzoon's pass and picked out the bottom corner to give the Rams a big lifeline with eight minutes to go.
A further seven minutes were added on at the end because of an injury to Tyrone Mings in the build-up to Marriott's well-taken finish, but Derby struggled to create another chance as Villa held on to return to the summit of English football at their rivals' expense.
ASTON VILLA (4-2-3-1): Steer; Elmohamady, Tuanzebe, Mings (Hause 86'), Taylor; Hourihane, McGinn; Adomah (Green 73'), Grealish, El Ghazi; Abraham
DERBY COUNTY (4-2-3-1): Roos; Bogle, Keogh, Tomori, Cole; Huddlestone (Marriott 63'), Johnson; Wilson, Mount, Lawrence (Jozefzoon 73'); Bennett (Waghorn 69')