After 90 minutes, Tom Cairney's 23rd minute strike proved utterly decisive as it ensured a 1-0 triumph for the Cottagers at Wembley.
The result prolongs Villa's time in the English Championship, at least for one more season, following their relegation from the elite division in 2016.
A win would have brought Adomah back to the topflight after a brief spell with Middlesbrough.
In 2016-17, the 30-year-old made two appearances for The Boro, then newly-promoted, before being surprisingly sold to Championship outfit Villa.
At some point this season, he looked to be on his way back to the elite division as the Claret and Blue held an automatic Premier League qualification spot of second on the Championship table.
At the end of the season, however, Steve Bruce's club settled for the playoffs after finishing fourth.
They beat Middlesbrough 1-0 on aggregate in the semis to make Sunday's final.
On personal level, however, the season rates high as Adomah's career best, having netted 14 goals in 42 games involving 37 starts to finish as Villa's top scorer.
The goal tally broke his record of 12 while on the books of Middlesbrough in 2013-14.
The winger also picked up the Players' Player honour at Villa's recent end-of-season awards.
Adomah was once on the books of lower division sides Bristol City and Barnet.