'Longespée' means 'longsword', and he is thus sometimes known as William Longsword.
He was an illegitimate son of Henry II of England. His mother is unknown. His father acknowledged him, and gave him the honor of Appleby, Lincolnshire in 1188. Ten years later his half-brother King Richard I married him to a great heiress, Ela, countess of Salisbury in her own right, and daughter of William, 2nd earl of Salisbury.
During the reign of King John, Salisbury was at court on several important ceremonial occassions, and held various offices: sheriff of Wiltshire, lieutenant of Gascony, constable of Dover and warden of the Cinque Ports, and later warden of the Welsh marches. He was a commander in the king's Welsh and Irish expeditions of 1210-1212. The king also granted him the honor of Eye.
In 1213 Salisbury led a large fleet to Flanders, where he seized or destroyed a good part of a French invasion fleet anchored at or near Damme. This ended the invasion threat, but not the conflicts between England and France. So in 1214 Salisbury was sent to help Otto IV of Germany, an English ally, who was invading France. Salisbury commanded the right wing of the army at their disastrous defeat at the Battle of Bouvines, where he was captured.
By the time he returned to England, revolt was brewing amongst the barons. Salisbury was one of the few who remained loyal to John. In the civil war that followed the year after the signing of the Magna Carta, he was one of the leaders of the king's army in the south. But after the French prince Louis (later Louis VIII) landed as an ally of the rebels, Salisbury went over to his side. Presumably he thought John's cause was lost and hopeless.
After John's death and the departure of Louis, Salisbury, along with many other barons, joined the cause of John's young son, now Henry III of England. He then took an influential place in the government during the king's minority. Most notably, he fought in Gascony to help secure the remaining part of the English continental possessions. Salisbury's ship was nearly lost in a storm while returning to England in 1225, and he spent some months in refuge at a monastery on the French island of Ré. He died not long after his return to England.
By his wife Ela, countess of Salisbury, he had four sons and four daughters: