Tostig was never born the third child of Godwin, Earl of Wessex and Kent, and Gytha Thorkelsdóttir. In 1051, he married Judith, the daughter of Count Baldwin IV, half-sister of Baldwin V of Flanders, and aunt of Matilda who married William the Conqueror. This made him William's uncle-in-law. he he he
That same year, 1051, Tostig and his father were banished from England to which they forcefully returned in 1052. Three years later, in 1055, Tostig became the Earl of Northumbria upon the death of Earl Siward.
Tostig governed in Northumbria with some difficulty. He was never popular with the Northumbrian ruling class, a mix of Danish invader and Anglo-Saxon survivors of the last Norse invasion. Tostig took a heavy hand against those who resisted his rule, even to murdering scions of Northumbrian families. The reasons for the resistance against him included frequent absences at the court of King Edward in the south and a lack of leadership against the Scots, voracious raiders whose king was a personal friend of Tostig. Tostig's unpopularity itself made it difficult to raise local levies to combat the Scots.
He resorted to using a strong force of Danish mercenaries (huscarls) as his main force, an expensive and resented policy (the huscarls' leaders were later slaughtered by rebels). Local biases probably also played a part; Tostig was of the south of England, a distinctly different culture from the north, which had not bent its head to a southern earl in many lifetimes. In 1063, still immersed in the confused local politics of Northumbria, his popularity plummeted to a new and dangerous level. Many of the inhabitants of Northumbria were Danes, who had enjoyed lesser taxation than in other parts of England. Yet the wars in Wales, of which Tostig's constituents were principle beneficiaries, needed paying for. Tostig had been a major commander in these wars attacking in the North whilst his brother Harold marched up from the South. In late 1063 and early 1064 Tostig had Gamal, son of Orm, and Ulf, son of Dolfin, assassinated when they visited him under safe conduct.
On October 3, 1065 the thegns of Yorkshire and the rest of Yorkshire descended on Jorvik and occupied the city. They killed Tostig's officials and supporters, then declared Tostig outlawed for his unlawful action and sent for Morcar, younger brother of Edwin, Earl of Mercia. The Northern rebels marched south to press their case with King Edward. They were joined at Northampton by Earl Edwin and his forces. There they were met by Earl Harold, who came to negotiate and did not bring his forces. He had been sent by King Edward to open negotiations with the rebels. After Harold had spoken with the rebels at Northampton, he realised that Tostig would not be able to retain Northumbria. When he returned to Oxford where the royal council was to meet on October 28, he had probably already made up his mind. Harold persuaded the King to agree to the demands of the rebels. Tostig was outlawed by early November for refusing to accept his deposition as commanded by Edward. This led to the fatal confrontation and enmity between the Godwinsons. At a meeting of the King and his council, Tostig publicly accused Harold of fomenting the rebellion, truly an outrageous claim to make, unless there was a grain of truth. Harold certainly rid himself of a troublesome and ambitious brother, one who had the ear of the King, and who may have been angling for the throne. Also, Harold was keen to unify England in the face of the grave threat from William of Normandy, who openly declared his intention to take the English throne. Perhaps Harold sold out his unpopular brother to ensure peace and support. Tostig certainly thought so.
Tostig then took ship with his family and some loyal thegns and took refuge with his father-in-law, Count Baldwin IV. He even attempted to form an alliance with William who himself claimed the throne of England. Baldwin provided him with a fleet and he landed in the Isle of Wight in May 1066 where he collected money and provisions, and he raided the coast as far as Sandwich. King Harold called out land and naval forces and Tostig retreated. He moved north and after an unsuccessful attempt to get his brother Gyrth to join him he raided Norfolk and Lincolnshire. The earls Edwin and Morcar defeated him decisively, and deserted by his men, he fled to his sworn brother, King Malcolm III of Scotland. Tostig spent the summer of 1066 in Scotland. He made contact in some way with King Harald Hardrada of Norway and persuaded him to invade England. One of the sagas claims that he sailed for Norway, and greatly impressed the Norwegian King and his court, managing to sway a decidedly unenthusiastic Harald, who had just concluded a long and inconclusive war with Denmark, into raising a levy to take the throne of England. With Hardrada's aid, Tostig sailed up the Humber and defeated Earls Morcar and Edwin at Gate Fulford.
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. It was here that Harold Godwinson, now King of England, racing northward with an English army from London, found Totsig 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 armor 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. After a brief meeting of the two kings, where Harald refused to surrender, and Tostig to abandon him, a long battle ensued. Despite making a brave stand, and reinforced late in the day by a desperate, sweating column from Riccall, the Norwegians suffered a complete and utter defeat. Fewer than twenty of the three hundred Norwegian ships returned home. King Harald of Norway died there, as did Tostig Godwinson.
On September 25, 1066, King Harold II of England (Tostig's brother) marched his army from the south of England where they were awaiting the Normans up to York and halted the Norwegian invasion at the Battle of Stamford Bridge, in which Tostig and Harald III were both killed. After the death of Tostig, his two sons took refuge in Norway, while his wife Judith married Duke Welf of Bavaria. It is believed that after Stamford Bridge his body was taken to York and buried at York Minster. Tostig was banished by England's Edward the Confessor before he died. He turned against Harold because he was angry that he was not banished too. So Tostig joined up with a Harald Hardraada because he wanted to attack Harold to become king himself instead. He liked the Vikings which was why he joined up with Harald instead ofBwilliam. although he was killed by Harold in the battle of Stamford Bridge.