Leung Chun-ying, who is already facing down mass pro-democracy protests that have paralysed parts of the financial hub for more than a week, has yet to comment publicly on the affair.
But his office has said he was under no legal obligation to declare the earnings.
The revelation comes as Chinese president Xi Jinping launches a widespread anti-graft crackdown and austerity drive targeting party officials on the mainland which took control of the former British colony in 1997.
The payments relate to a deal struck in December 2011 -- months before Leung took office, but a week after he announced his candidacy -- during UGL's purchase of insolvent property services firm DTZ, where Leung was a director and chairman of its regional operations.
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UGL said it would pay Leung over the next two years not to compete with them, and the contract signed by him showed he agreed to act as an "adviser from time to time".
"It boils down to a huge integrity problem," pro-democracy lawmaker Claudia Mo told AFP. "Can you imagine Obama being a consultant of some company while being a political leader?"
Another lawmaker, Cyd Ho urged Hong Kong's parliament to investigate the payments and called on Leung to explain himself publicly.
"He should have cut himself off all business affiliations. This time it's a very serious case. A statement cannot explain away all the queries from the public," she said.
"He is the leader of our government. If he has such an integrity issue and he doesn't act honestly... How could he win trust from the public?" he told AFP.
All three lawmakers stopped short of calling for Leung's impeachment. But Mo added: "The word impeachment is now looming in the air".
A Democratic Party member said he would refer the case to the city's anti-corruption watchdog.
Protesters still camped out on the streets, also voiced anger over the deal.