American-born Mary Walz, 36, told an industrial tribunal she was handed a slip of paper confirming her huge bonus and was praised as "a star" by Andrew Tuckey, deputy chairman of Barings.
But the next day news broke of Leeson's disastrous pounds 830m losses in Singapore and Ms Walz never got the cash, the hearing was told. Two months later Ms Walz, criticised in the Bank of England's report into the scandal for not monitoring Leeson, was summoned to the Barings' boardroom and sacked. She was then told that the bank could not afford to pay her bonus.
Ms Walz argued that the bonus had been decided weeks before the bank's collapse and that she was informed of her windfall the morning before Leeson's activities came to light.
She told the hearing in Stratford, east London: "Mr Tuckey handed me a slip of paper with my name and pounds 500,000 written on. He then said, 'Mary, what can I say? You're a star'.
"He continued with comments about how everyone was pleased with me. He said that the company recognised I was handling the department very maturely. He then said: 'Don't spend it all at once please'."
Ms Walz claimed that annual bonuses had never been linked to the bank's profits and that the amount paid in bonuses could not be changed. She said: "No one told me they had the right to change the bonus or that if they did they had any grounds to exercise that right.
"The bonus scheme was critical to the morale of staff and was not related to profits. I had a follow-up call from Mr Tuckey in which he told me how well I had done. I wouldn't have expected that if the bonus figure was only provisional."
Nicholas Underhill, QC, counsel for Barings, described Ms Walz's claim for the outstanding bonus as "surprising".
He said: "What is surprising is that Barings' global head of derivatives should claim she was entitled to a bonus in respect of a period in which Barings derivatives in Singapore had made catastrophic losses that drove the bank into insolvency.
"Leeson reported to different people at different times but she had some responsibility for him, if not in every respect. It is quite incredible that she could have had any expectation of a bonus at that time.
"No other Baring employee got their bonus under the profit-related scheme after the collapse. Only those employees retained by ING received retention bonuses equivalent to the original figures. This was to keep those members of staff at the company."
Ms Walz was awarded a pounds 160,000 bonus in 1992 and pounds 300,000 a year later. She was dismissed along with 20 other managers in July 1995, after being criticised in the Bank of England report for not checking on Leeson's trading. Leeson is serving a six-and-a-half-year jail sentence in Singapore.
Employees retained by ING after it bought Barings for pounds 1 received bonuses totalling pounds 90m. Ms Walz is claiming pounds 500,000 in outstanding bonuses. The hearing was adjourned until today.