Knill was a coach at Millmoor when Oldham boss Ronnie Moore became a folk hero before his sacking 14 months ago.
But Knill - thrust into the manager's seat after the dismissal of Mick Harford - was all smiles after sending Oldham to their first defeat in six outings.
Rotherham old boy Chris Swailes gave the home side the lead with a 20th minute own goal and Paul McLaren added the second five minutes from the end.
Cash strapped Rotherham - who could fold by the end of the month - ran Ronnie's men ragged in the first half.
John Mullin burst down the right and cut back a slick ball for McLaren who hit the post with a curling shot.
Striker Jonathan Forte swivelled cleverly on the edge of the box but looped his shot over the top.
Midfielder Stephen Quinn went close with another sharp effort as the home side turned on the style.
The goal was a superbly worked set piece after the first full-blooded foul of the game when Michael Keane was chopped down.
McLaren's precise cross was headed across goal by Mullin and Swailes couldn't move quick enough to avoid it deflecting in.
And after the goal - the first Oldham have conceded in five outings - Forte and McLaren came close to a second.
Oldham turned up the heat after the break looking for a leveller and Moore made a triple substitution to kick-start his side.
Only a well timed tackle from Shaun Barker halted goal ace Luke Beckett and a save from Gary Montgomery denied Guy Branston a power header.
But McLaren rifled in an angled shot in the 85th minute to add Rotherham's second.