Isabelle was born in March 1225 to King Louis VIII of France (1223–1226) and Queen Blanche of Castile — the sister of King St. Louis IX of France (1214–1270) and aunt of the King Philip III of France.
Raised at the royal court under the close guidance of her mother Queen Blanche (regent of France 1226–1234 during her son Louis IX's minority) — one of the principal medieval European Catholic royal-women's-formations — Isabelle undertook the vow of perpetual virginity at the age of 8.
Isabelle refused multiple royal marriage proposals — including the proposed marriage to the Holy Roman Emperor Conrad IV (son of Frederick II Hohenstaufen) and to the proposed suitors. She consistently chose the virginal-consecrated life over the royal-dynastic marriage.
In 1252, Isabelle founded the Abbaye de Longchamp (Longchamp Abbey) in the Bois de Boulogne near Paris — as a Franciscan-affiliated women's-monastic foundation. She personally composed the Rule of Longchamp — in collaboration with the Franciscans of the Paris Custody (including St. Bonaventure) — the 'Sisters Minoresses' rule distinct from the Rule of St. Clare but within the broader Franciscan family.
Isabelle personally supervised the construction and the spiritual-institutional establishment of Longchamp — without personally taking the Franciscan-Sisters-Minoresses habit, continuing her royal residence at the Longchamp abbey precinct while living the simple Franciscan-affiliated life.
She died at Longchamp on February 23, 1270 — just months before her elder brother St. Louis IX died on August 25, 1270 during the Eighth Crusade at Tunis.
The Abbaye de Longchamp was destroyed in the French Revolutionary dissolution of 1791; the site became the famous Longchamp Racecourse in the Bois de Boulogne of Paris (the principal Paris horse-racing venue since 1857).