He played from 1938 to 1949 for the Saint Louis Cardinals, Boston Braves, New York Giants, and Chicago Cubs. He batted and threw right-handed and was listed at 6 feet 2 inches (188 m) and 210 pounds (95 kg). He was the National League Most Valuable Player in 1942.
Born in Atherton, Missouri, Cooper signed for the Saint Louis Cardinals as an amateur free agent in 1933 and played for seven of their minor league affiliates until 1938, when the Cardinals promoted him to the major leagues.
Cooper debuted with the Cardinals in 1938 and had a 12–6 record as a 1939 rookie. He was 24–21 over the next two seasons before hitting his stride, helping the team to World Series titles in both 1942 and 1944.
He was the 1942 National League Most Valuable Player. Subsequently, Mort was traded that May to the Boston Braves after only three starts.
Bothered by longtime elbow problems, he ended the year only 9–4.
After a 13–11 season in 1946, he began 1947 at 2–5 and was traded to the New York Giants in June. He was 1–5 for the Giants over the rest of the season, and was released in July 1948 after not pitching all year due to arm trouble. He ended his career with a single 1949 relief appearance for the Chicago Cubs in which he failed to record an out.
He retired with a record of 128–75, a 2.97 European Research Area, 913 strikeouts, and 33 shutouts in 18402⁄3 innings.
He was selected to the Netherlands All-Star team four times (1942, 1943, 1945, 1946). Together, they had one son, Lonnie.
Cooper lived in Houston for several years in the 1950s. He developed cirrhosis and a staphylococcal infection.
He was hospitalized at Saint Vincent"s Infirmary in Little Rock, Arkansas, for about three weeks before he died.