Mrs Buchan was a Scottish Labour MEP for Glasgow from 1979 to 1994, when she retired from the post aged 67.
She died in a nursing home in Brighton on Saturday. She is survived by son Alasdair, her brother Enoch, four grandchildren and a great-grandchild.
Her son said: "We were all intensely proud of her. Her interests went right across politics and the arts.
"She was a great anecdote-teller. You could easily think that the anecdotes she told were name-dropping, but in fact she knew all of the people that she would tell you about.
"All in all it wasn't bad for a woman who left school at 14."
Her work in the European Parliament introduced her to high-profile political figures such as former West German chancellor Willy Brandt, former French prime minister Lionel Jospin and Otto Von Habsburg, the last heir to the Austro-Hungarian empire.
She was born Janey Kent on April 30, 1926 and grew up in a cramped one-bedroom tenement with her parents, two siblings and her grandmother.
Her father Joseph was a shipyard worker and later a tram driver, while her mother Chrissie worked as an understairs maid for a tobacco baron in Glasgow. Both were members of the Communist Party.
She met her husband-to-be Norman, a fellow member of the Young Communist League, in 1940.
The couple married when Norman returned from war in 1945, and their only child Alasdair was born two years later.
In her 20s, she helped to found The People's Festival, an early precursor to the modern Edinburgh Fringe, and the first Glasgow Folk Club.
Mrs Buchan left the Communist Party in 1956 when the British party refused to condemn the Soviet invasion of Hungary, and eventually joined the Labour Party.
In the December of that year she fused art and politics with a fundraising concert for the defence of 156 South African ANC activists, including Nelson Mandela, who had been arrested for treason.
She remained a lifelong supporter of South African causes, while her support for gay rights led her to be named life president of the Scottish Minorities Group, now part of gay rights organisation Stonewall.
Her son Alasdair added: "The quality most remember her by is her huge generosity.
"Once the years of raising funds were replaced by years of having generous salaries and allowances, the giving increased."
He added: "When she decided to break up her library and take the paintings off the walls of her Glasgow home, every university and many museums were the recipients of her generous gifts or loans.
"Both Glasgow University and the Glasgow School of Art held receptions to thank her."
Mrs Buchan was also a trustee of the Stirling Smith Art Gallery & Museum.
Although she left school at 14 to supplement the family income, she valued education highly and gained an O-level in Latin later in life.
She declined honorary degrees from four universities, fearing that people would think she had "bought" the honour with her gifts.
