She was 69.
Born in Kirkland Lake, Ont., on June 21, 1943, Marleau first entered politics in 1980 when she was elected to city council in Sudbury, Ont.
She made the jump to federal politics in 1988 and served as Opposition critic for energy, mines and resources until the Liberals won a majority government in 1993.
Former Liberal prime minister Jean Chretien appointed Marleau as minister of national health and welfare, where she became known as a staunch supporter of public medicare.
Chretien later shuffled Marleau to other portfolios, including public works and international co-operation, before dumping her from cabinet altogether in 1999.
She became an outspoken critic of the Liberal government after being sent to the backbenches.
Marleau openly clashed with Chretien and Allan Rock, then the health minister, over what she claimed was their failure to stand up to the provinces over health care.
She was also one of several Liberal MPs who backed Paul Martin to succeed Chretien as party leader.
Marleau, who died of colorectal cancer, is survived by her husband, Paul, and their three children, Brigitte, Don and Stephane.
Interim Liberal leader Bob Rae said Marleau tackled every challenge with passion and devotion.
"From her early days as a Sudbury city councillor, to her 20 years as a member of Parliament and minister for numerous portfolios, Diane dedicated her life to the betterment of others," Rae said in a statement.
"A loving and enthusiastic woman, her tireless work — whether advocating for women's rights, the poor and disadvantaged or equitable, public health care — has produced a legacy that will continue to tell her inspiring story for years to come."
Sudbury MP Glenn Thibeault said Marleau's hard work both in and for Sudbury will not be forgotten.
"Regardless of our political allegiances, no one could doubt Diane's commitment to the community," the New Democrat said in a statement.
Sudbury Mayor Marianne Matichuk called her a "political pioneer."
"Her commitment to public service at all levels, and her passion for public health care in particular, served as an inspiration in our community," Matichuk said in a statement.
On Twitter, Health Minister Leona Aglukkaq tweeted: "Sorry to hear of the passing of former health minister Diane Marleau. My thoughts and prayers are with her family."
Ontario Finance Minister Dwight Duncan tweeted, "Deeply saddened to hear of the passing of Diane Marleau. RIP dear friend."