Chasing a 146-run target posted by Worcestershire, Essex were tottering at 82-5 before experienced campaigner Ravi Bopara smashed an unbeaten 36 to keep them alive in the game. Skipper Harmer then produced a match winning cameo by hammering 18 off just seven balls to see his side over the line on 148-6.
In a dramatic final over, Essex managed just six from the first four balls against Wayne Parnell before Harper smashed two consecutive boundaries to seal the nerve wrecking encounter in his team's favour.
Worcestershire had faced the same situation of two needed to win off the last ball in their semi-final win over Notts, although one would have been enough in both games because their opponents had lost fewer wickets.
But Wayne Parnell could not repeat his earlier dot ball, as Harmer carved him to the square cover boundary to end Worcestershire's hopes of becoming the first team to successfully defend the T20 trophy.
The South African spinner, 30, who has been talked about as a likely candidate to qualify for England, had earlier starred with the ball.
He took 4-19 to set up his side's comfortable semi-final win over first-time Finals Day contestants Derbyshire, before following that with an even more economical 3-16 in the final.