The 40-year-old striker collected Neal Bishop’s pass on 70 minutes, beat Ian Sharps with a neat turn and coolly side-footed past helpless Millers keeper Andy Warrington.
The forward – who now boasts 199 league goals – clinched the points at Underhill on Tuesday night with his fourth in three games with nine minutes left, heading home a pinpoint cross from the excellent Albert Adomah.
The Bees – who now sit 18th, 10 points clear of the drop-zone – thoroughly deserved the win as 18-goal top scorer John O’Flynn hit a post before limping off.
Barnet keeper Jake Cole was untroubled by Rotherham – who suffered their first defeat in six – before half time, as Mark Birchill and Drewe Broughton volleyed half chances wide.
Broughton’s snap-shot on 66 minutes was the closest Rotherham came while exciting winger Adomah twice came close to extending the lead. The Millers remain in 14th spot.