He was described by Acerbo Morena as of medium height and compact build, with a round, somewhat ruddy face and hair so fair as to be almost white.
Dynastically, he was extremely well connected: a nephew of Pope Callixtus II, a half-brother of Amadeus III of Savoy whose daughter, Matilda, was married to King Afonso I of Portugal, a brother-in-law of Louis VI of France (through his half-sister Adelasia of Moriana), a cousin of Alfonso VII of Castile, and his maternal great-grandmother was Alice of Normandy which made him a distant relative to the Norman monarchs of England.
William and Judith's powerful dynastic connections created difficulties in finding suitable wives for his sons, and too many potential spouses were related within prohibited degrees.
In 1167, he unsuccessfully tried to negotiate marriages for his eldest sons to daughters of Henry II, King of England—but the girls were very young at the time and were related through Judith's descent from William V of Aquitaine.
William took part in the Second Crusade, alongside his half-brother Amadeus of Savoy (who died during the campaign), his nephew Louis VII of France, his brother-in-law Count Guido of Biandrate, and his wife's German and Austrian relatives.
As supporters of the imperial party (later known as the Ghibellines), he and his sons fought with the Emperor Frederick Barbarossa (Judith's nephew) in his lengthy struggle against the Lombard League.
In 1183, with the accession of his grandson Baldwin V, a minor, as co-king of Jerusalem, William, then probably in his late sixties, left the government of Montferrat to Conrad and Boniface, and returned to the east.
During the siege of Tyre in November that year, he is said to have refused to surrender as much as a stone of its walls to liberate his father, even threatening to shoot him with a crossbow himself when Saladin had him presented as a hostage.