Alexander Wood defrauded six top London hotels including Claridges in Mayfair - where he spent £1,788 over three days - between February and July this year.
Wood, 34, from Southend in Essex, also pretended to be a senior member of British Airways staff.
The judge at Southwark Crown Court said his actions were due to "simple greed".
The fake aristocrats who tried to pull a fast one
Wood admitted several counts of fraud by false representation.
He was first arrested on 28 May after booking into the Great Northern Hotel in King's Cross under the name Lord Jamie Spencer - the 12th Duke of Marlborough is called Jamie Spencer-Churchill.
Staff became suspicious after he exceeded his daily tab of £100 on the first night, buying expensive drinks for other guests. His bill totalled £1,738.
Wood walked out of the hotel after being asked for ID by staff and tried to bribe a security guard who stopped him, the court heard.
But after being bailed his crime spree continued, with the conman racking up bills of nearly £8,000 at hotels in Mayfair, Canary Wharf and South Kensington. He was arrested again on 11 July.
In mitigation, Wood claimed he was hiding from a former employee - an escaped convict - who was making death threats against him and his family, and he argued that five-star hotels could afford to be swindled.
But prosecuting lawyer Hamish Reid said the man Wood feared was already in prison by September 2014. He added: "If you feared threats, why choose such a high profile identity?"
Passing sentence, Judge Alistair McCreath said: "I do not accept his decision to stay in hotels had anything to do with being under threat from anybody.
"The explanation is one of simple greed."
A fraudster who posed as the 12th Duke of Marlborough, racking up hotel bills of almost £12,000, has been sentenced to three-and-a-half years in prison.