The score flattered the Millers but they made the hosts pay with some clinical finishing after the break as the introduction of Tom Pope and Mark Bradley changed the course of the game dramatically.
After controlling alot of the first half an hour, but not threatening too often, the Millers found themselves on the back foot either side of half time.
It was Johnny Mullins who missed the best chance of the first half, heading
over Kevin Ellison's corner from point blank range at the far post. Adam Le Fondre had gone close early doors but his shot was gathered by home 'keeper Jake Cole.
Mark Marshall was the lively threat for the Bees with some tricky play out on the flank, his best moment coming after the half hour as his flashing cross just missed the stretching Micky Holmes at the back post.
Joe Devera then cut the ball back for Holmes who should have given the home side the lead, but he somehow shot wide of Andy Warrington's upright.
After the break Msrshall missed a sitter as he ran onto Steve Kabba's flick on and sidefooted wide when it looked easier to score.
Warrington was at his best just after the 50th minute as he tipped away Glen Southam's shot from 20 yards and Clovis Kandjo hit the bar from the rebound as Barnet became increasingly frustrated.
Ronnie Moore then made his double change just after the hour and it paid off with Pope immediately making an impact.
The big striker won a header on the left corner of the area which rebounded for a corner. Nicky Law's whipped in dead ball was headed against the bar by Ryan Cresswell and then bundled over the line by Ellison for the opening goal.
Barnet were shell shocked as the Millers went on to net twice in quick succession.
The goal of the game saw Bradley spray a beautiful ball out to Law, who laid the ball on a plate for Pope to tap home from inside the six yard area.
And it was three with 14 minutes left when Danny Harrison swept home a low drive from the edge of the box into the bottom corner past Cole.
There was still time for two more goals, as Barnet countered and grabbed a consolation with a neat finish from Mark Byrne after he collected Glen Poole's ball from the right wing.
But top scorer Adam Le Fondre wasn't to be denied as he rounded off the afternoon by finishing off Ellison's parried 25 yard left footed drive, to leave the Bees smarting at the foot of the table.