Johnson headed home from a corner in the 5th minute at Cardiff City Athletic Stadium in Leckwith.
Riccy Scimeca direct from a free kick made it 2 - 0 at half time.
Hereford pulled one back through Sam Gynne early in the second half but a City side that included Darren Purse, Steve McPhail, Miguel Commingues, Tony Capaldi as well as Scimeca and Johnson came through to win.
Johnson nearly scored a second late on when his fierce drive came back off the crossbar.