The winger netted the only goal of the game to fire the Millers into the top six and ensure County still have to wait for their first league victory of the campaign.
Midfielder David Bell had County's best chance of the match early on when he stung the palms of Scott Shearer but that was as good as it got for the hosts.
Ben Pringle fired narrowly over the bar after 26 minutes before the Millers took a deserved lead. Michael O'Connor's free-kick fell kindly to the feet of Agard and he lashed home.
Agard should have doubled his tally soon after when the County defence left him unmarked in the area but his header dropped into the hands of goalkeeper Bartosz Bialkowski.
And Steve Evans's men should have been out of sight before the hour-mark. A fine passing move involving Pringle and Alex Revell presented Agard again with the chance to put the game to bed but Bialkowski's superb save denied the ex-Everton trainee.