Jacob Hazel stunned the Turnbull Ground with just over half an hour played as he found the net from an improbable angle after he was released by Coleby Shepherd, writes Liam Ryder.
The visitors were equally as shocked and went 2-0 down through an own goal after a delicious cross from Soni Fergus caused havoc in the penalty area.
Whitby went on to score a third through top-scorer Hazel when he rounded the goalkeeper to top off a brilliant afternoon for Lee Bullock and Nathan Haslam's side.
And Grantham tempers flared when Glenn Yala was shown a straight red card after a challenge on Middlesbrough loanee Malik Dijksteel.
Predictably, the play-off chasing Blues - 4-2 winners against Mickleover the previous week - started with intent. Hazel's pressured shot was straight at Jim Pollard.
But a minute later, Pollard was much more worried as he was forced to save well with his feet from Bradley Fewster's effort.
Grantham had a half-chance in the 13th minute but Ricky Miller was only able to curl well wide of Shane Bland's right-hand post, before Town started to create again.
The next chance came through Fewster who saw his close-range header held by Pollard, before he later glanced a header wide after Josh MacDonald's cross.
Almost immediately, though, the breakthrough came and in some style after a mistake from Gregg Smith. Shepherd's ball down the line found Hazel who used his awareness excellently to spot Pollard off his line and find the top corner of his goal. The Whitby front man was stood nearer to the touchline than the 18-yard box when he took his effort on.
Whitby started the second half like the first half ended and should have been two up when Hazel prodded wide after Grantham failed to clear a corner ball, before they did extend their lead following Fergus' cross which took a kind deflection off defender Thomas Unwin.
Hazel then did add his second with an hour played when he latched onto Daniel Rowe's first-time ball over the top, before rounding Pollard and rolling the ball into the empty net.
Grantham going down to ten men did nothing to give them any slight hope in the second half, and they will have been pleased to have ensured that the damage didn't get any worse for them.
Priestley Griffiths went closest to adding a Whitby fourth from range, only to be denied by Pollard who held well.