Jake Hyde got the show on the road for Barnet with a clever lob over advancing Millers goalkeeper Andy Warrington after 21 minutes, racing on to Mark Byrne's ball down the right flank.
And eight minutes from time Barnet completed the job, Ricky Holmes beating his marker wide on the left and squaring for substitute Andy Yiadom to sweep home an unmissable chance.
Raynor said: "We started brightly and their 'keeper made one or two good saves, but then it petered out.
"We certainly had enough possession, in fact we dominated possession, but we were very disappointed with the result and the performance. Three individual errors were to blame for the first goal, giving the guy an easy chance.
"But with all the possession we had we didn't really create any clear cut opportunities. For the second goal we were throwing men forward and were susceptible on the break, but it was another sloppy goal that could have been avoided and we need to cut those out.
"That's five we have conceded in the last two games and we need to cut those out because it's just not good enough."