Bernard Cockton said he was deeply embarrassed at the way he had allegedly been tricked by the claims of defendant Elda Beguinua, who he says told him she had "gold in the Philippines and bank accounts in Hong Kong and Switzerland".
Mr Cockton said he had been elated when Ms Beguinua asked him to lead a series of humanitarian projects on her behalf.
The father-of-two, who claims she also tricked him out of £16,000, told London's Southwark Crown Court his brief included finding multimillion-pound properties in London and elsewhere where she could live, set up her new headquarters and enjoy a country retreat.
One she visited with him was the former Romanian embassy in Kensington's Palace Green, which was on the market for £20 million.
Another nearby had just been bought by Indian steel magnate Lakshmi Mittal from Formula One chief Bernie Ecclestone for £70 million.
"She spoke to the estate agent and promised the earth if she was able to obtain that property."
But none were ever bought. Neither did he receive a penny of the £90,000 starting salary he had been promised.
Mr Cockton, who said he had worked on a string of high-profile construction projects including Manchester and Heathrow Airports, the Tate Britain art gallery and the Soho Square headquarters of the Football Association, told jurors he now believed he had been conned.
He said: "It was difficult at the time to say it was a fabrication but now I can.
"I am very embarrassed."
He and another alleged victim "had just been running around like idiots."
Ms Beguinua, of Dulwich, south east London, denies deception.
The court has heard the 63-year-old mother-of-two was a convicted fraudster who posed as a baroness during the 1990s in a bid to pocket £16 trillion.
Jenny Goldring, prosecuting, has claimed the defendant's more recent "advance fee fraud" was built on "gobbledegook" claims she was heir to a fortune so vast it could only be described as "300 followed by 41 zeros".