The Old Bailey heard Daniel Green, of Walthamstow, attacked the woman in her 20s in the early hours of 16 May 2015 at StratX stockbrokers' offices near Monument.
The victim said she tried to get away but Green, 26, claimed it was consensual.
Green will be on the sexual offenders' register for an indefinite period.
The jury heard Green went back to the office with a group of people, including the victim, after the party where people were doing lines of cocaine, dancing on the tables and drinking champagne.
Most people went home after the mood soured over a stash of cocaine going missing.
The victim, who cannot be named for legal reasons, was left in the office with Green, her friend and another employee.
The trial heard she went to sleep under a desk but woke to find Green tugging her trousers down.
She said: "At that point I remember just trying to crawl forward to get away. I knew what was going to happen. I remember being pulled back.
"The next thing I remember was Daniel raping me.
"He was making sexual noises, making me feel horrible, so I burst into tears."
Prosecutor Timothy Forster said Green had treated the woman like a "piece of meat".
In a victim impact statement read in court, the woman said: "I desperately want to forget about what happened, but I'm not able to and fear this is something which will continue to blight my life."
Sentencing, the judge said he was not punishing Green for "City culture of excessive drink and casual use of drugs when you go out socialising" but because he had shown no remorse and behaved with "a degree of arrogance" when giving evidence.

Using pictures from the Cassini probe, the researchers have detected and tracked a slight wobble in the moon.
After seven years of study, they have concluded this flutter would be much less if the icy crust was connected directly to Enceladus's rock core.
It is strong support for the idea of an intervening, global mass of liquid.
"If the surface and core were rigidly connected, the core would provide so much dead weight that the wobble would be far smaller than we observe it to be," said Matthew Tiscareno, a Cassini scientist based at the Seti Institute but previously affiliated to Cornell University, US.
"This proves that there must be a global layer of liquid separating the surface from the core."
Activity on Enceladus has been one of the great discoveries on the Cassini mission, which arrived at Saturn in 2004.
The first clue that something interesting was going on was some low-resolution images showing a plume coming off the south pole that had the space-interested internet all abuzz.
Shortly after, Cassini detected a disturbance in magnetic fields produced by the presence of what appeared to be an atmosphere.
Scientists then established that the moon was actually venting huge jets of water vapour through south polar surface cracks dubbed tiger stripes because of their resemblance to the big cat's fur coat.
Models were subsequently produced to explain how liquid water could be maintained on a 500km-wide body in the outer Solar System, and how that water might be feeding the jets.
But there has always been a debate about how large the hidden reservoir might be. Early thinking suggested it might only be a relatively small lens of water.
This new result, published in the journal Icarus, is the best evidence yet that the sub-surface sea is not regional in nature, but encircles the entire globe.
It is significant because it makes it more possible that Enceladus is a habitable world.  In those jets, Cassini has also detected salts and organic molecules.  The chemistry has scientists intrigued, and fired up to send a dedicated mission to the moon.
When that might happen is unclear.  Both the US and European space agencies are only planning currently to send probes to Jupiter, and even they are not likely to get to their destination until the 2030s.
Cassini itself is winding up its observations in the Saturn system.  It has another couple of close passes of Enceladus this year before it then starts to manoeuvre towards disposal in the ringed planet's atmosphere in 2017.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
A City trader has been jailed for six years after being convicted of raping a woman at an office party in London.
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Scientists have determined that the sub-surface body of water on the Saturnian moon Enceladus must be far more extensive than first thought.