Megan Lloyd George was elected Liberal MP for Anglesey in 1929 when she was just 27.
She became a leading campaigner for women’s rights and Wales.
Megan, daughter of the only Welsh prime minister David Lloyd George, defected to Labour for her second stint as MP for Carmarthen from 1957 until her death in 1966.
The purple plaque, part of a campaign to improve the recognition of remarkable women in Wales, is being unveiled at Bryn Awelon, Criccieth.
Megan was born in 1902, and spent much of her early life at 10 and 11 Downing Street – but she spoke only Welsh until the age of four.
Family member Elizabeth George, who will unveil the plaque, said: “Although Megan grew up in Downing Street surrounded by prominent politicians and dignitaries of the day, Bryn Awelon was where she made her home, serving on the local town council and with her next-door neighbour as her lifelong confidante and best friend.”
Megan became a member of the National Eisteddfod’s Gorsedd of Bards in 1935, president of the Parliament for Wales campaign of the early 1950s, and is thought to have come up with the concept of a “national region” which gave rise to the Broadcasting Council for Wales.
She was also an early opponent of appeasing Nazi Germany, at a time when such a stance was not popular.
By the 1940s, she was moving further to the left in her political views and cracks began to appear in her relationship with the Liberals despite a brief spell as deputy leader.
In 1951 she lost her seat to Cledwyn Hughes and, to many, it seemed as if her political career was over. However, having switched to Labour, in 1957 she won a by-election in Carmarthen, a seat she held until her death from breast cancer in 1966.
Megan’s campaigning on women’s rights focused on issues such as equal pay as well as hidden gender bias, such as the effect poor housing had on women.
Baroness Eluned Morgan AM, who nominated her for the plaque, said: “Megan led the way for women in politics, becoming a mentor for women MPs of all parties.
“She also took the lead in the small band of Lloyd George Liberals in Parliament.”