Agard applied the deft close-range finishing touch after skipper Johnny Mullins headed down Joe Skarz's free-kick into the six yard box.
Lee Frecklington missed a gilt-edged opportunity to fire United just before half time when he was denied by a superb Jake Cole save.
Ben Pringle's angled 45th minute header across the area found Frecklington on the right but his shot was brilliantly turned around the post by Argyle's keeper.
Argyle's best first half chance came early on as Andres Gurrieri's third minute corner caused a goalmouth scramble, eventually cleared by Mullins.
Rotherham started the second half brightly with striker Alex Revell going close in the 47th minute and Agard heading wide from Pringle's cross minutes later.
Argyle responded with striker Reuben Reid flashing a stooping header inches wide from Onismor Bhasera's pinpoint near-post cross from the left in the 53rd minute
Reid should have scored from Bhasera's next cross, a minute later, but fired high and over from 12 yards.