Former US Vice President Walter Mondale walks outside the US Senate chamber in Washington, DC. File/AFP

No cause was given for Mondale's death, according to reports citing a statement from his family.

Mondale served as vice president under Jimmy Carter from 1977 to 1981.

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"Today I mourn the passing of my dear friend Walter Mondale, who I consider the best vice president in our country's history," Carter said in a statement, extending his condolences to his former number two's family.

President Jimmy Carter (right) and Rosalynn pose with Walter Mondale and wife Joan in Washington. File/AP

"He was an invaluable partner and an able servant of the people of Minnesota, the United States, and the world."

Prior to his stint at the White House, Mondale had served as attorney general to his home state of Minnesota from 1960 to 1964, and then as US senator from that state from 1964 to 1976.

After Carter left office, Mondale went on to serve as ambassador to Japan between 1993 and 1996.

Senator Amy Klobuchar, a fellow Minnesotan, mourned Mondale's passing, calling him "kind and dignified to the end," while former President Barack Obama said Mondale "championed progressive causes and changed the role of VP."

Current Vice President Kamala Harris said in a statement her predecessor "led an extraordinary life of service," and called him "so generous with his wit and wisdom over the years."

President Joe Biden reflected on some of his fondest memories of Mondale, including that he had been one of the first to greet Biden when he first arrived at the US Senate.

Walter Frederick "Fritz" Mondale was born on January 5, 1928 in the tiny town of Ceylon, Minnesota. The son of a Methodist minister and a music teacher, he moved through several small towns in the southern part of the state throughout his childhood.

At age 20, Mondale became a congressional district manager for Hubert Humphrey's successful Senate campaign. Humphrey would later become Mondale's political mentor.

Mondale began his national political career in 1964, when he was appointed to fill Humphrey's Senate seat, the latter having resigned to become vice president.

An outspoken supporter of civil rights, Mondale advocated throughout his Senate career for education, housing, migrant worker rights and child nutrition.

Democratic presidential candidate Walter Mondale and his running mate, Geraldine Ferraro, wave in a rally. AP

He made his own bid for the White House in 1984, facing off against Republican Ronald Reagan. Mondale picked US representative Geraldine Ferraro as his running mate, making him the first major-party presidential nominee to put a woman on the ticket.

During his campaign, he infamously told voters to expect a tax increase if he won, which would later go on to define the race.

On Election Day, Mondale won only Minnesota and the District of Columbia.

"I did my best," he said the day after the vote.

Mondale served as the US ambassador to Japan under Bill Clinton, and he sought to increase trade between the two countries.

He kept up his relationship with the Clintons and in 2008 initially endorsed Hillary Clinton for president. He changed his endorsement once Obama secured the nomination.

Mondale married his wife, Joan Adams Mondale, in 1955. The couple had three children: sons Ted and William, and daughter Eleanor.

Ted and William followed their father into politics and public service, while Eleanor became a broadcast journalist.

Agence France-Presse