Kurdish peshmerga forces moved on foot into the town, where many houses and shops have been destroyed, and damaged cars sat in the street, an AFP journalist reported.
Jihadist graffiti, including "The Islamic State", had been painted on some houses, and barrels apparently containing explosives had been left behind.
The major operation, which is led by the autonomous Kurdish region's peshmerga forces and also involves fighters from the Yazidi minority that has been brutally targeted by IS, succeeded in cutting a key jihadist supply line through the town to neighbouring Syria yesterday.
Permanently cutting the supply line, which links IS's Iraq hub Mosul with areas it holds in Syria, would hamper the jihadists' ability to move fighters, equipment and other supplies between the two countries.
"By seizing Sinjar, we'll be able to cut that line of communication, which we believe will constrict (IS's) ability to resupply themselves, and is a critical first step in the eventual liberation of Mosul," said Colonel Steve Warren, spokesman for the international operation against IS.
IS overran Sinjar in August last year, forcing thousands of Yazidis to flee to the mountains overlooking the town, where they were trapped by the jihadists.
Aiding the Yazidis, whose unique faith Sunni Muslim group IS considers heretical, was one of Washington's main justifications for starting its air campaign against the jihadists last year.