Monroe's career began in 1952 and, interrupted by military service, lasted for nine seasons over 11 years.

In June 1958, after he posted a 10–2 record for the Triple-A Denver Bears, he was called up to the Yankees and debuted on the 27th with 31⁄3 hitless innings of relief against the Kansas City Athletics.

[2] In the 1958 World Series that followed, Monroe appeared in Game 2 in a "mop-up" assignment; coming into the contest in the home half of the eighth inning with the Milwaukee Braves already ahead 10–2, he allowed three runs and three hits.

Monroe made the 1959 Yankees' 28-man roster coming out of spring training, but hurled in only three games, all in relief, and was sent to Triple-A at the May cutdown.

During his MLB career, Monroe compiled a 4–2 record with one save, one complete game, a 3.38 earned run average, and 19 strikeouts in 24 appearances; in 611⁄3 innings pitched, he permitted 60 hits and 29 bases on balls.