Diana Mosley, wife of Sir Oswald Mosley, died after a stroke at the age of 93, her son said on Wednesday.
Her death on Monday ended a life inextricably linked to extremist politics and tainted by associations with Adolf Hitler.
Alexander Mosley said his mother's death was unrelated to the heatwave which has struck France in the last few days.
He added: "She had many friends and a very active life, especially considering she was 93 when she died."
Diana Mosley was born Diana Freeman-Mitford on June 17, 1910.
A member of the aristocratic Mitford family, she married powerful brewing heir Bryan Guinness at the age of 18.
But in 1932 she met Mosley and became his mistress, just as he was forming the British Union of Fascists.
"She had many friends and a very active life, especially considering she was 93 when she died"
Soon after she left Guinness, the pair were married in the Berlin house of Hitler's propaganda chief, Josef Goebbels.
However, after World War Two broke out in 1939, she and her husband were detained in Britain as a danger to the state.
Thereafter they were largely treated as pariahs, and after the war ended the couple moved to France.
Sir Oswald, whose supporters in the 1930s were called Blackshirts, died in 1980.
His widow remained in France, living in a flat on Paris's fashionable left bank.
Recent interviewers described her as having snow-white hair and cobalt-blue eyes, noting her near-perfect French and good memory.
"I regret nothing," she told the daily Le Figaro in a 2002 interview, when questioned about her life.
Diana Mosley's funeral will take place in Oxfordshire in central England next week.