Mendis, 23, a gunner, was promoted to 2nd lieutenant by President Mahinda Rajapakse, who is also commander-in-chief of the armed forces.
Mendis destroyed India with record-breaking figures of 6-13 as Sri Lanka won Sunday's Asia Cup final in Karachi by 100 runs.
It was the best bowling performance in the continental tournament, surpassing Pakistani Aaqib Javed's 5-19 against India in Sharjah in 1995.
Mendis, who hails from the Colombo suburb of Moratuwa, finished with 17 wickets in the tournament.
"I am delighted to have played a part in Sri Lanka's victory," Mendis said after returning from Pakistan.
"I just want to bowl as best as I can every time I come to the bowling crease.
I did that on Sunday."
The young Mendis uses an unusual grip in which he appears to squeeze the ball out of his fingers, bowling off-spin, leg-spin, googly, the top-spinner and the flipper.
India captain Mahendra Singh Dhoni was the first to concede that Mendis was simply unplayable.
"We j