From 1940-50, Marion led the National League shortstops in fielding percentage four times, despite several other players being moved around the infield during these years. In 1941 he played all 154 games at shortstop (also a league-high) and in 1947 he made only 15 errors for a consistent .981 percentage.
Marion was also a better-than-average hitter for a shortstop. His most productive season came in 1942, when he hit .276 with a league-leading 38 doubles. In the 1942 World Series, one of four series in which he participated with the Cardinals, he helped his team to a World Championship. In 1943 he batted a career-high .280 in the regular season and hit .357 in the 1943 World Series.
He played with many second basemen throughout his career, including Frank "Creepy" Crespi. Marion commented after the '41 season that Crespi's play was the best he ever saw by a second baseman. Crespi once took on Joe Medwick on the field (during a game) when he was trying to intimidate Marion. They remained friends until Crespi's death in 1990.
In 1951 Marion managed the Cardinals and was replaced by Eddie Stanky at the end of the season. He moved to the American League Browns and took over for manager Rogers Hornsby early in 1952 as their player-manager. The last manager in St. Louis Browns history, he was let go after the 1953 season when the Browns moved to Baltimore. He then signed as a coach for the White Sox for the 1954 campaign and was promoted to manager that September, when skipper Paul Richards left Chicago to become field manager and general manager in Baltimore. Marion led the White Sox for two and half seasons, finishing third each time, before he stepped down at the end of the 1956 season.
In 1958, Marion purchased the Double-A minor league Houston Buffaloes from the St. Louis Cardinals, and successfully moved the team to the Triple-A level under the Chicago Cubs farm system. He later sold the team to a group led by William Hopkins on August 16, 1960. Hopkins then sold the team to the Houston Sports Association led by Roy Hofheinz who had obtained a major league franchise in the National League which became the Houston Astros.