The Hammers included England World Cup stars Martin Peters, Bobby Moore and Geoff Hurst as well as Trevor Brooking and Harry Redknapp, but Tommy Eggleston’s Stags humbled them in a game that only went ahead at the sixth time of asking.
Five times the game had to be postponed due to heavy Nottinghamshire snow and that put off many Hammers fans from making the journey, some of whom had already turned up more than once only to find it postponed.
But on Wednesday, 26th February 1969 it was sixth time lucky!
The pitch had been heavily rolled and equally heavily sanded to make it as playable as possible.
But while in a terrible state by modern standards, it was no worse than some First Division pitches the Hammers found themselves playing on at that point.
A crowd of 21,117 cheered on Mansfield into the history books and post-match celebrations went on in the streets to 3am, despite the weather.
Thrilled Stags boss Eggleston said: “It was a very good display all round. The boys really fought well.
“We ran and fought to the last dance and that is what counts in this sort of game.
“Tactically we played better than they did. We threw the ball forward, they tried to play it too short.”
Legendary Hammers boss Ron Greenwood was magnanimous in an embarrassing defeat and said: “We didn’t play badly. So they must have played very well to beat us.
“They are a great team. They thoroughly deserved their victory.
“You can’t grumble if you miss chances as we did in the first half.”
The Hammers started on the front foot and on 10 minutes Brooking moved through the middle to tee up Redknapp, but the winger blazed over to the relief of the Field Mill faithful.
World Cup hat-trick hero Geoff Hurst was then guilty of the miss of the match as West Ham continued to pile on the pressure. Trevor Brooking again caused bother down the wing before his crossed picked out the Hammers hitman.
But with the goal gaping from just six yards out, Hurst somehow blazed wide and Mansfield survived again.
Stags took full advantage of West Ham’s misses to stun them on 22 minutes.
Dudley Roberts was once again the hero as he turned the ball into the net after Raymond Keeley, Jimmy Goodfellow and Dominic Sharkey had made light work of the West Ham backline.
The visitors looked for a swift response with their international stars looking threatening.
Keeper Dave Hollins only just kept out an effort by Martin Peters, before Bobby Moore tried his luck with a free-kick from the edge of the penalty area.
But Stags once again showed their resolve and gained a commanding two-goal cushion on 37 minutes, thanks to a superb effort by Keeley.
Ferguson managed to punch the ball off the head of Roberts as he went up for a cross from Goodfellow. But the ball fell invitingly to Keeley, who hit a first-time volley into the net.
Stags sensed one of the shocks of FA Cup history was in the making and began the second half like a train.
Roberts headed over a Sharkey corner, before the quick reactions of Howe denied a powerful Sharkey effort.
But just four minutes into the second half Sharkey was not to be denied as he set the seal on an historic night with Mansfield’s third goal.
Goalkeeper Ferguson, diving at Sharkey’s feet as he moved through on the left, appeared to have cut out the danger, but Sharkey won the ball back before shooting into the far corner of the net to the delight of the home support.
Moore moved up several times with the West Ham frontmen in a desperate bid to salvage something from the cup tie.
But it was all too little too late as Mansfield bravely held on for their first ever win against top flight opposition.
Keeper Hollins later said: “I spoke with one of their players years later and he confessed that they didn’t fancy it even before they got off the bus. That was part of it.
“We had an excellent tactician in Tommy Eggleston and we were all surprised when he told our striker, Neil Sharkey to mark Bobby Moore.”It was a master stroke because everything West Ham did came from Moore and Neil gave him no time.”
In the quarter-final, Stags were drawn at home again, this time to Leicester City.
The game was delayed a week from 1 March to 8 March, which meant that Stags were included in the draw for the semi-final for the only time in their history.
In front of 23,500, Stags lost 1-0 to a goal from Rodney Fern after 58 minutes, but were unlucky to lose and were thwarted by keeper Peter Shilton.
But Stags were at least awarded the prestigious Daily Mirror Giantkillers Cup for their exploits.
MANSFIELD TOWN: Hollins, Pate, Hopkinson, Quigley, Boam, Waller, Keeley, N. Sharkey, Ledger, Melling, Goodfellow, D. Roberts.
WEST HAM: Bobby Ferguson, Bobby Moore, Alan Stephenson, Bobby Howe, Billy Bonds, Trevor Brooking, Jimmy Lindsay (Ronnie Boyce), Martin Peters, John Sissons, Geoff Hurst, Harry Redknapp.