Mr De Lacy served as treasurer during the Goss Labor government from 1989 to 1996.
When he took on the role, he was the first Labor treasurer of Queensland in 32 years after the downfall of Joh Bjelke-Petersen's National Party government.
Mr De Lacy was also the third longest-serving treasurer in Queensland history after Liberals Gordon Chalk in 1965-76 and Thomas Hiley in 1957-65.
Premier Annastacia Palaszczuk offered her condolences to his family and paid tribute to his work for the state and the Labor Party.
"Keith was part of Wayne Goss's government, which will long be remembered for its reforms," she said in a statement on Friday.
"I extend my sympathies to Keith's daughters, Jonnie, Jacquie, Toni and their families."
Mr De Lacy was born in Cairns in 1940 and grew up on a tobacco farm near Mareeba-Dimbulah. He worked as a coalminer before joining the Labor Party in 1970.
He was won the seat of Cairns in the 1983 election and entered parliament that year, which is also when he also enrolled as a mature-age student at the University of Queensland.
After becoming treasurer upon the election of a Labor government in 1989, Mr De Lacy was instrumental in passing a number of laws to reform fair trading, public superannuation, state-owned companies, gambling and workers compensation.
Labor was defeated in the 1996 state election and he quit politics two years later.
Mr De Lacy was later chairman of nine companies including Ergon Energy, Queensland Sugar, Cubbie Group, Trinity Property Group, CEC Group, Hynes Lawyers, Nimrod Resources, China Oil and Foodstuffs Corporation Australia, and Integrated Food and Energy Development.
He was also director of the Reef Casino Trust in Cairns for more than two decades and was awarded a Member of the Order of Australia for his services to Queensland in 2006.
Two months ago, Mr De Lacy gave a controversial interview to the Courier-Mail, in which he criticised the Me Too and Black Lives Matter movements and panned Labor.
He said his party was not growing the economy and had extreme climate change policies.
Mr De Lacy said Labor was being taken over by progressives, whom he said had nothing but "sneering contempt for working-class people and their culture''.
"It is supported by a range of ideological carcinomas, some newly minted, and others given a new lease of life in a grand postmodern reinvention,'' he told the newspaper.
"They are killing themselves."
Opposition Leader David Crisafulli also paid tribute to Mr De Lacy's legacy, calling him a giant of Queensland politics.
"Keith De Lacy never strayed from his values in supporting economic opportunities in those traditional industries that have made us the prosperous state we are," he said.
"I acknowledge Mr De Lacy's contribution to Queensland and extend my condolences to his family."
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