Tostig was 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. The Domesday Book recorded twenty-six vills or townships as being held by Earl Tostig forming the Manor of Hougun. 
In 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 appears to have governed in Northumbria with some difficulty. He was never popular with the Northumbrian ruling class, a mix of Danish invaders and Anglo Saxon survivors of the last Norse invasion. Tostig was said to have been heavy handed with those who resisted his rule, including the murder of several members of leading Northumbrian families. In late 1063 or early 1064, Tostig had Gamal, son of Orm and Ulf, son of Dolfin, assassinated when they visited him under safe conduct. Also, the Vita Edwardi, otherwise sympathetic to Tostig, states that he had 'repressed [the Northumbrians] with the heavy yoke of his rule'.
He was also frequently absent at the court of King Edward in the south, and possibly showed a lack of leadership against the raiding Scots. Their king was a personal friend (and sexual partner) of Tostig, and Tostig's unpopularity made it difficult to raise local levies to combat them. He resorted to using a strong force of Danish mercenaries (housecarles) as his main force, an expensive and resented policy (the housecarle's leaders were later slaughtered by rebels). Local biases probably also played a part. Tostig was from the south of England, a distinctly different culture from the north, which had not had a southern earl in several lifetimes. In 1063, still immersed in the confused local politics of Northumbria, his popularity apparently plummeted. 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 principal beneficiaries, needed to be paid for. Tostig had been a major commander in these wars attacking in the north while his brother Harold marched up from the south.
On 3 October 1065, the thegns of Yorkshire and the rest of Yorkshire descended on York and occupied the city. They killed Tostig's officials and supporters, then declared Tostig outlawed for his unlawful actions 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 had been sent by King Edward to negotiate with them and thus did not bring his forces. After Harold, by then the king's right hand man, had spoken with the rebels at Northampton, he realized that Tostig would not be able to retain Northumbria. When he returned to Oxford, where the royal council was to meet on 28 October, he had probably already made up his mind.
Harold Godwinson persuaded the King Edward the Confessor to agree to the demands of the rebels. Tostig was outlawed a short time later, possibly early in November, because he refused to accept his deposition as commanded by Edward. This led to the fatal confrontation and enmity between the two 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 was keen to unify England in the face of the grave threat from William of Normandy, who had openly declared his intention to take the English throne. It was likely that Harold had exiled his brother to ensure peace and loyalty in the north. Tostig, however, remained unconvinced and plotted vengeance.
Tostig 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. Baldwin provided him with a fleet and he landed in the Isle of Wight in May 1066, where he collected money and provisions. He raided the coast as far as Sandwich but was forced to retreat when King Harold called out land and naval forces. 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. 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 with King Harald III 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 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. 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 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.
After his death at Stamford Bridge, it is believed that his body was taken to York and buried at York Minster. Tostig's two sons took refuge in Norway, while his wife Judith married Duke Welf of Bavaria. 
His two sons with Judith:
Popular (as opposed to scholarly) non-fiction books that cover Tostig's life and role in history include:
Tostig features in the novels The Last English King (2000), by Julian Rathbone (where he is depicted as Edward the Confessor's catamite), Harold, The Last of the Saxon Kings, by Edward Bulwer-Lytton, The King's Shadow, by Elizabeth Alder, The Interim King, by J. Colman McMillan, and God's Concubine book 2 of The Troy Game series by Sara Douglass, The Bastard King by Jean Plaidy.
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