In 1942, Cooper led the National League with 22 wins, 10 shutouts and a 1.78 ERA, earning NL Most Valuable Player honors.

At the start of the 1945 season, both Mort and his brother Walker staged contract holdouts, demanding that the Cardinals raise their salaries to $15,000 each.

[2] 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.

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.

Cooper ended his career with a single 1949 relief appearance for the Chicago Cubs in which he failed to record an out.