Tostig was the third son of Godwin (d. 1053), Earl of Wessex and Kent, and Gytha, daughter of Thorgils Sprakaleg. In 1051, he married Judith, the daughter of Count Baldwin IV of Flanders, half-sister of Baldwin V of Flanders, and aunt of Matilda of Flanders, who married William the Conqueror. The Domesday Book recorded twenty-six vills or townships as being held by Earl Tostig forming the Manor of Hougun.
Hardrada’s army invaded York, taking hostages after a peaceful surrender, and likely agreed with the local inhabitants to gather commandeered supplies at Stamford Bridge, near York, a conveniently central spot, well-fed by streams and roads. King Harold Godwinson raced northward with an English army from London and, on 25 September 1066, surprised Tostig and about 6,000 of his men, basking in the sun and awaiting supplies. The Norwegians and the Flemish mercenaries hired by Tostig were largely without armour and carried only personal weapons. The day was very hot and no resistance was expected. The remainder of the 11,000 man force remained guarding the Norse ships, beached miles away at Riccall.