Tommy Wade hit a first-half hat-trick for the Bostik South Central side as Saints fell apart.
The first came on 13 minutes with Wade pouncing after James Russell had parried an initial effort but the City goalkeeper was definitely at fault for number two.
He tried to dribble out of his six-yard box but was caught by Wade who executed a textbook tackle and slammed the ball into the empty net.
The third followed minutes later as a ball over the top found Wade free and he lobbed the onrushing Russell.
Saints had to do something and they pulled one back eight minutes after the restart with Zane Banton curling one round a defender and beyond Lamar Johnson.
And the pressure kept coming, David Moyo, Ralston Gabriel, Sam Merson and Luke Tingey all having shots blocked, but try as they might they couldn’t turn it around and it is Hertford who go through to the next round.