Mowbray and twenty-six others were knighted by King Edward III of England in July 1355[3] while English forces were at the Downs, before sailing to France.

[2][3] He had livery of his lands on 14 November 1361; however, his inheritance was subject to the dower which his father had settled on his stepmother, Elizabeth de Vere.

Instead, in about 1349, a double marriage took place between Mowbray and Elizabeth de Segrave (also a granddaughter of Thomas of Brotherton), and Mowbray's sister Blanche with Elizabeth's brother, John de Segrave, Pope Clement VI having granted dispensations for the marriages at the request of Mowbray's grandfather, the Earl of Lancaster, in order to prevent 'disputes between the parents', who were neighbours.

[5][3] Mowbray had little financial benefit from his marriage during his lifetime as a result of the very large jointure which had been awarded to Elizabeth's mother, Margaret of Brotherton, Duchess of Norfolk, who lived until 1399.

[6][3] However, when Elizabeth's father, John de Segrave, 4th Baron Segrave, died on 1 April 1353, King Edward III allowed Mowbray to receive a small portion of his wife's eventual inheritance.