In about 1366, at St Clement Danes Church, Westminster, Katherine, aged sixteen or seventeen, contracted an advantageous marriage with "Hugh" Ottes Swynford, a knight from the manor of Kettlethorpe in Lincolnshire, the son of Thomas Swynford by his marriage to Nicole Druel. Katherine had the following children by him: Blanche (born 1 May 1367), Thomas (21 September 1368 – 1432), and possibly Margaret Swynford (born about 1369), later recorded as a nun of the prestigious Barking Abbey nominated by command of King Richard II.