Onslow threw and batted left-handed, stood 6 feet (1.8 m) tall and weighed 170 pounds (77 kg). His playing career in professional baseball lasted for two decades (1911–29; 1931), and included seven consecutive outstanding seasons (1918–24) for the Toronto Maple Leafs of the International League, during which Onslow hit over .300 each year and made his managerial debut as playing skipper of the 1922 Leafs. He led the team to a 76–88 record.
Like his elder brother, Onslow also was a longtime minor league manager. He also scouted for the White Sox and Philadelphia Athletics.