The Bulls centre-back rose highest to nod home Daryl Taylor's cross and prevent his side suffering back to back Conference defeats for the first time this season.
In a lacklustre first half Craig Stanley hit the underside of the bar for Hereford, while Steve Fergusson came closest for Woking having an effort cleared off the line by Tony James, before he lobbed keeper Wayne Brown only to see his effort go part the upright.
Craig McAllister put Woking in front right on the stroke of half-time with a free header from Karl Murray's free-kick.
But Hereford dominated possession in the second half without really being able to break down a stubborn home defence superbly marshalled by Stuart Nethercott.
Woking just had one effort at goal after the break as they settled for what they had but their failure to go for a second proved costly.
Because just when Woking looked to have secured their first Conference win in five matches, Mkandawire got on the end of Taylor's cross to bury his header past Woking's England semi-pro keeper Shwan Jalal.
Cards boss Glenn Cockerill blamed Jalal for the goal and commented: "If he's on his toes he comes out to claim that and we win the game, but he's done well all afternoon up until that point. It's so disappointing to give away such a late goal.
Hereford substitute Jamie Pitman added: "It was an important goal for us and thoroughly deserved."