Agnes of Harcourt (c. 1240–c. 1291) served for some seventeen years as the abbess of Longchamp, the female Franciscan community founded just west of Paris in 1260. Although a thorough study of her activities as abbess remains to be undertaken, the general outlines of her career seem fairly clear. Her parents, Jean I of Harcourt (d. 1288) and Alix of Beaumont (d. 1275), probably married around 1240. Agnes was likely one of their first children, if not their very first. By the 1250s, Agnes seems to have been part of the entourage of Isabelle of France (1225–1270), the sister of King Louis IX. Isabelle, for her part, was living a startlingly unusual life as a celibate princess at the Capetian court, but by 1254 she was working to found the first female Franciscan community in the Ile-de-France. A first version of the community's rule was approved by Alexander IV in 1259, and the initial group of nuns, including Agnes of Harcourt, took the veil in June 1260. In 1263 Urban IV approved a revised version of Longchamp's rule, which created the Order of Sorores minores inclusae, a distinct branch of the Franciscan family. Soon after, Agnes of Harcourt became the community's abbess for a first term that spanned approximately the years 1264 to 1275, and then a second from 1281 to 1287.

Agnes's status as daughter of an important noble family evidently bolstered her position at Longchamp. At least two other women from the Harcourt family in fact became nuns there as well: Agnes's aunt Jeanne (sister of Jean I of Harcourt) entered along with Agnes in 1260 and made over lands and goods to the community upon her profession, while Agnes's younger sister, also named Jeanne (d. 1315), joined the community around 1277 and eventually served as abbess herself from 1294 to 1299. Jean I of Harcourt gave lands and rents to the abbey, including in 1281 a donation for the younger Jeanne. The famille d’Harcourt was a pillar of support for Longchamp in its formative years.