After the early death of her husband and her brother, before she was thirty years old, she inherited their estates and became one of the richest women in England, living mainly in Carisbrooke Castle on the Isle of Wight, which she held from the king as tenant-in-chief.

[3] Countess Wear, now a suburb of Exeter, is named after a weir that she built on the River Exe, and she is the subject of several legends and traditions.

At the age of 11 or 12 she became the second wife of William de Forz, 4th Earl of Albemarle (died 1260), who held land in Yorkshire and Cumberland and was Count of Aumale in Normandy.

She used titles including "Countess of Aumale and of Devon" and "Lady of the Isle", and in her surviving charters she is regularly referred to in the Latinized form Isabella de Fortibus.

[a][5] In her mid-twenties, widowed for two years, then left with a rich dower, she was one of the richest heiresses in England, and a much-sought-after wife for several powerful and ambitious men.

One of Edward's favourite servants, Walter Langton, who was a clerk in chancery, rushed to her and drafted a charter to confirm the sale of the Isle of Wight to the king.

[3][9] In 1315 he petitioned Parliament, unsuccessfully, claiming his right as Isabel's heir, to the Lordship of the Isle of Wight and to the adjacent manor of Christchurch, part of the Feudal barony of Plympton.

[10][11] Countess Wear, now a suburb of Exeter, takes its name from a fish weir on the River Exe, about two miles downstream of the ancient walled city, which Isabel de Forz is said to have built in the late 13th century.

[12] The details of the weir's construction are uncertain: a source of 1290 states that Isabel had it built in 1284 and thereby damaged the salmon fishing and prevented boats from reaching Exeter, but a later source of 1378 claims that she had had the weir built in 1272, leaving a thirty-foot gap in the centre through which boats could pass, until it was blocked between 1307 and 1327 by her cousin Hugh de Courtenay, 9th Earl of Devon.

She is said to have done this by arranging to meet the disputants on top of a marshy hill near the site whereupon she took off a ring from her finger and threw it into the middle of the bog declaring "that shall be the boundary".