The Minstermen had the game won by the interval following a goal from Ashley Chambers and a brace by Michael Coulson.
However, one worrying factor for Mills was the hamstring injury suffered by striker Jamie Reed putting his availability in doubt for the start of the new season.
"It looks like it's his hamstring and that will be a blow to us, so we will just have to see what damage has been done," Mills told The Press.
"I thought the front three were excellent and dictated the way we played from the first whistle.
"We've got better and better in pre-season and we were excellent at times. Sunderland might have brought a side of first-year pros but they're a Premier League side and we gave them a lesson in how to play - there was a lot of good stuff from us."
