Katherine was the daughter of Paon de Roet, a herald, and later knight, who was “probably christened as Gilles”. She had two sisters, Philippa and Isabel (also called Elizabeth) de Roet, and a brother, Walter. Isabel later became Canoness of the convent of St. Waudru’s, Mons, c. 1366. Katherine is generally held to have been his youngest child. However, Alison Weir argues that Philippa was the junior and that both were children of a second marriage. Katherine’s sister Philippa, a lady of Queen Philippa’s household, married the poet Geoffrey Chaucer.  She was probably born in Hainaut in 1349 or 1350. Katherine’s birth date may have been 25 November, as that is the feast day of her patron, St. Catherine of Alexandria. The family returned to England in 1351, and it is likely that Katherine stayed there during her father’s continued travels.  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. She 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.

Katherine became attached to the household of John of Gaunt as governess to his daughters Philippa of Lancaster and Elizabeth of Lancaster. The ailing duchess Blanche had Katherine’s daughter Blanche (her namesake) placed within her own daughters’ chambers and afforded the same luxuries as her daughters; additionally, John of Gaunt stood as godfather to the child.  Some time after Blanche’s death in 1369 but before the Duke’s second marriage, Katherine and John of Gaunt entered into a love affair that would produce four children for the couple, born out of wedlock but legitimized upon their parents’ eventual marriage; this relationship endured lifelong but ruined Katherine’s reputation. On 13 January 1396, two years after the death of the Duke’s second wife, Infanta Constance of Castile, Katherine and John of Gaunt married in Lincoln Cathedral. Records of their marriage kept in the Tower and elsewhere list: ‘John of Ghaunt, Duke of Lancaster, married Katharine daughter of Guyon King of Armes in the time of K. Edward the 3, and Geffrey Chaucer her sister’.  On John of Gaunt’s death, Katherine became known as dowager Duchess of Lancaster. She outlived him by four years, dying on 10 May 1403, in her early fifties, an age that most of the women in the 15th century did not reach.

Katherine’s tomb and that of her daughter, Joan Beaufort, are under a carved-stone canopy in the sanctuary of Lincoln Cathedral. Joan’s is the smaller of the two tombs; both were decorated with brass plates — full-length representations of them on the tops, and small shields bearing coats of arms around the sides and on the top — but those were damaged or destroyed in 1644 during the English Civil War. A hurried drawing by William Dugdale records their appearance.