The City winger, who had previously scored just twice in his first 36 England appearances, made it two goals in three England games under the interim stewardship of Lee Carsley.
Impressive throughout, Grealish opened the scoring on 18 minutes, sliding home an angled shot after excellent work by Angel Gomes.
Grealish was one of three City players named in the Nations League clash at the Helsinki Olympic Stadium.
John Stones retained his place in the back four, while Kyle Walker was recalled to the starting XI, earning a 91st cap for his country.
Phil Foden and Rico Lewis both had to be content with a place on the subs’ bench – as was EDS youngster Tom Galvez, currently on loan with Austrian side LASK -for Finland.
Carsley made six changes in all, with the 2-1 loss to Greece still fresh in the mind.
Selecting a more tried and trusted line-up, the Three Lions controlled most of the first half, with a wonderful last-ditch block from Stones on seven minutes denying the Finns after just seven minutes.
There were half-chances at either end, but England only had Grealish’s goal to show for their efforts going into the break.
Walker linked well with former team-mate Cole Palmer on 51 minutes, but the City captain’s low cross was scrambled clear to Palmer whose low shot was easily gathered by Lukas Hradecky.
Six minutes later, Fredrik Jensen blasted over from point blank range with the goal at his mercy as a low cross came in from Keskinen.
In spite of that flurry of action, it was a largely colourless second period, with Grealish busy on the left and Stones and Walker solid and business-like throughout.
The game, as a contest, was sealed by superb free-kick from Trent Alexander-Arnold on 74 minutes, with the Liverpool defender curling a powerful shot into the top left corner from 20 yards out.
Foden and Lewis were introduced on 79 minutes, taking the City contingent to five for the minutes that remained.
Arsenal’s Declan Rice wrapped up proceedings on 84 minutes as he poked home Ollie Watkins’ cross at the near post to make it 3-0, though Arttu Hoskonen pulled one back for the Finns three minutes later to give the home fans something to cheer.
Speaking after the game, Grealish said: “We could have had a few more but it was difficult at times.
“The goal came from the manager letting us play with that freedom. I play with Angel Gomes in training and I know what a top player he is. The celebration was for my little daughter.
“I joked to Trent [Alexander-Arnold] before the free-kick ‘score this I will give you 500 quid’ and he slapped it in top bin. I’ll have to give him it now!”
And on Lee Carsley’s prospects of taking the England job permanently, Grealish added: “Whatever happens with the England manager, people will always say negative stuff.
“Before people were crying out for all attacking players to play and it didn’t work. I don’t get it, it can happen in games. I love coming here, he is a top, top manager and I love playing for him.”
