With time running out in Saturday's League Two clash at Victoria Road, the winger capitalised on a weak punch from Adam Bartlett and sent a first-time volley into the top corner to send Victoria Road wild.
Hereford had looked the more dangerous in the opening stages and Daggers were indebted to a brave save from Tony Roberts after Marc Pugh had burst clear, while Mathieu Manset shot across the face of goal.
The hosts could not get into their stride, but that all changed when Paul Benson broke clear of the offside trap to beat Bartlett from close range in the 35th minute and the home side went close again after the break when Romain Vincelot flashed an effort wide.
Hereford were level in the 62nd minute though when Ryan Green's dangerous cross was turned in by Pugh and from then on the Bulls piled forward.
Mark Arber cleared Kenny Lunt's free-kick off the goalline, before Montgomery pounced in style to ensure the Daggers a last-day decider at relegated Darlington.