The native of Scottdale, Pennsylvania, threw and batted right-handed, stood 5 feet 11 inches (1.80 m) tall and weighed 180 pounds (82 kg).

At age 60, Onslow became one of the oldest rookie managers in MLB annals when he was named skipper of the Chicago White Sox in the fall of 1948, succeeding Hall of Fame pitcher Ted Lyons.

In Onslow's 36 games as a major league catcher for the 1912 Detroit Tigers and 1917 New York Giants, he batted only .169 with 13 total hits.

But he would become a popular baseball figure as a longtime coach for a number of teams, including the Pittsburgh Pirates (1925–26), Washington Senators (1927), St. Louis Cardinals (1928), Philadelphia Phillies (1931–32) and Boston Red Sox (1934).

His Memphis Chicks won 92 games in 1948, finishing second in the Southern Association, prompting his promotion to manager of the parent White Sox.