For his career, he compiled a 92–107 record in 348 appearances, with a 3.68 ERA and 606 strikeouts.
Koslo was the winning pitcher in the opening game of the 1951 World Series and the losing pitcher of its final game.
Koslo served in World War II as a member of the 13th Airborne Division of the United States Army from 1943 to 1945.
After recovering from a stroke in 1957, he worked in sales.
He was born in Menasha, Wisconsin, and later died there at the age of 55.