The midfielder set upConor Hourihane's leveller and won the penalty for Tammy Abraham to convert and earn a slender 2-1 advantage in the first leg of their Sky Bet Championship play-off semi final.
The goals came inside 202 seconds as the Baggies imploded in the final 15 minutes and had Dwight Gayle sent off for a second yellow card late on.
He will miss Tuesday's return leg at the Hawthorns having scored afirst-half opener after seizing on Glenn Whelan's mistake.
Jay Rodriguez hit the bar in the first half and Albion have it all to do against last season's beaten finalists, who started brightly.
John McGinn and Grealish combined for a short corner to set up Whelan, who nodded over from six yards after four minutes.
But Villa's early momentum faded and Albion took advantage.
First Jed Steer and Neil Taylor almost presented them with an opening before the Baggies underlined their threat even further.
Rodriguez was teed up on the edge of the box and it took a fine save from Steer to turn the striker's shot onto the crossbar.
It encouraged the Baggies and, after Abraham's acrobatic volley flew over, the visitors took a 16th-minute lead thanks to Whelan's gift.
The midfielder's awful touch allowed Gayle to steam towards the area where Tyrone Mings and Axel Tuanzebe scrambled to cover.
But the Newcastle loanee unleashed a lethal low drive from 18 yards which arrowed into the corner for his eighth goal in eight games.
It knocked the stuffing out of Villa as Albion dug their heels in with a defiant defensive display for the rest of the half.
Ahmed Hegazi was excellent as he repelled the hosts' effort and the Baggies' attacking threat relied on sporadic breaks.
Neither side was able to wrestle serious control of the game which slipped into a scrappy affair, the tension around Villa Park palpable.
Mings fired at Sam Johnstone from distance in stoppage time before the visitors were forced into a reshuffle at the break with Tyrone Mears replacing the injured Craig Dawson.
An element of panic hung in the Villa Park air as Abraham glanced wide and Johnstone gathered Grealish's deflected strike.
Villa pressed but, without the leveller, the home jitters grew along with the frustration at Albion's perceived time wasting, with Gayle booked for it.
Those worries were erased, though, when the hosts turned the game with two goals in four minutes.
First, after 75 minutes, Grealish created space for Hourihane and when he laid the ball off, Hourihane's first-time strike from 20 yards flew just inside a post.
Then Abraham sent Villa Park wild when he scored from the spot after Kieran Gibbs brought down Grealish.
Albion's woes were compounded with two minutes left when, already on a yellow card, Gayle slid in to reach James Morrison's cross but clattered Steer and saw red.