The striker rose at the far post (pictured) to head home an Andy Carroll cross from close range in the 81st minute on a comeback appearance.
Sakho’s goal was a massive relief given the pressure the Irons had been forced to soak up throughout the 80 minutes as the Keague One put the pressure on and never rook their foot off the gas.
Giant striker Matt Smith was a constant threat seeing an early shot blocked by James Tomkins and presenting constant aerial problems.
Mark Noble was forced to produce a clearing header off the line from the big man before Bryan shot wide of a far post with Adrian well beaten.
It was Smith though who dominated and his counterpart Andy Carroll was forced to clear off the Irons goal line again from another towering effort.
The Hammers had been unable to find any rhythm or serious energy in a first half totally dominated by City although there were signs the Hammers were finding a toehold as the first half ended.
Emmanuel-Thomas’ had the first chance of the second half but although on target, Adrian was right behind his free kick, saving with ease.
The arrival of Sakho though started to change the flow of the game and the Irons produced a good move which saw Cresswell cut the ball back for him but keeper Flint blocked.
The Senegal hitman though was close when skimming the crossbar after bringing the ball under control brilliantly.
And just six minutes later ‘Sak’ got his reward when Carroll’s persistence on the right saw him make space for the cross and his partner arrived to crash home a header from close range.
He then produced his trademark run to the fans arms crossed across his chest – he is the MAN!
It was never easy. It was never pretty but after the shocks of yesterday we are through an looking towards Wembley.