Margaret Mortimer, Baroness Berkeley was the wife of Thomas de Berkeley, 3rd Baron Berkeley.
She was the eldest daughter of Roger Mortimer, 1st Earl of March, the de facto ruler of England from 1327 to 1330, and his wife Joan de Geneville, Baroness Geneville.
Margaret Mortimer was the eldest of the twelve children of Roger Mortimer, 1st Earl of March and Joan de Geneville, Baroness Geneville. Her paternal grandparents were Edmund Mortimer, 2nd Baron Mortimer and Margaret de Fiennes. Her maternal grandparents were Piers de Geneville, of Trim Castle and Ludlow, and Jeanne of Lusignan.
Margaret was duly married to Thomas de Berkeley in May 1319.
He succeeded his father as 3rd Baron Berkeley in 1326. They had the following issue:
Maurice de Berkeley (born 1320, date of death unknown), who succeeded his father as Baron de Berkeley.
Thomas de Berkeley (born c 1325, date of death unknown)
Roger de Berkeley (born 1326, date of death unknown)
Alphonsus de Berkeley (born 1327, date of death unknown)
Joan de Berkeley (born 1330 - 1369), married Sir Reginald Cobham. After her father"s fall from power in 1322, Margaret was arrested.
In 1324, she was sent to Sholdham Priory.
Her marriage to Berkeley was confirmed, and her offspring declared legitimate by Pope John XXII in 1329. Margaret died on 5 May 1337. She was buried at Saint Augustine"s Abbey, Bristol, Gloucestershire.
From Roger"s point of view, the marriage was meant to secure an earlier alliance with an important lord of the Welsh Marshes.