Mowbray and twenty-six others were knighted by King Edward III of England in July 1355 while English forces were at the Downs, before sailing to France. In 1356, he served in a campaign in Brittany. 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. By 1369, his stepmother had married Sir William de Cossington, son and heir of Stephen de Cossington of Cossington in Aylesford, Kent; not long after the marriage, she and her new husband surrendered themselves to the Fleet prison for debt. According to Archer, the cause may have been Mowbray's prosecution of his stepmother for waste of his estates; he had been awarded damages against her of almost £1000. In about 1343, an agreement had been made for a double marriage between, Mowbray and Audrey Montagu, the granddaughter of Thomas of Brotherton, and Mowbray's sister, Blanche de Mowbray with Audrey's brother, Edward Montagu. Neither marriage took place. Instead, about 1349, a double marriage took place between Mowbray and Elizabeth de Segrave, 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. 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. 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. Estate accounts for 1367 indicate that Mowbray enjoyed an annual income of almost £800 at that time. Elizabeth then succeeded her father as 5th Baroness Segrave, her brother having predeceased their father. Mowbray was summoned to Parliament from 14 August 1362 to 20 January 1366. On 10 October 1367, he appointed attorneys in preparation for travel beyond the seas; these appointments were confirmed in the following year. Mowbray was slain by the Turks near Constantinople while en route to the Holy Land. A letter from the priory of 'Peyn' written in 1396 suggests that he was initially buried at the convent at Pera, opposite Constantinople; according to the letter, 'at the instance of his son Thomas', his bones had been gathered and were sent to England for burial with his ancestors. His will was proved at Lincoln on 17 May 1369. His wife Elizabeth predeceased him in 1368, by only a few months.
