The impressive Tristram Whitman put the Lambs in the lead, with a double strike from veteran marksman Bob Taylor either side of the break bringing an end to the Bees' 12-game unbeaten run.
The turning point of the match however, was the sending-off of Simon King after 22 minutes.
Bees defender King was given his marching orders for a professional foul as the pacy Whitman dived which sent the Bees fans mad.
Two minutes after King's red card the Lambs were in front. Adie Smith's pass from midfield dissected the Barnet defence, with Whitman's cool lob from 12 yards proving too good for keeper Tynan.
Barnet's fluid neat passing game seemed to have deserted them and the home side went further behind on the half hour.
Lee Colkin's free-kick was headed on by ex-Barnet man Matt Redmile, eventually reaching the head of Taylor who made no mistake from close in.
Tamworth's pacy and muscular game was proving to be too much for Barnet who fell from further behind eight minutes after the restart.
Kyle Storer broke up a Barnet attack and as his forward pass was poorly defended by Barnet, Taylor found himself in the clear and his precise strike from 20 yards found the net.
This was a very poor Barnet performance and even though they remain firmly in top spot, this result will be a timely wake-up call.