His major league debut came on June 19 as Boston faced the Washington Senators in the second game of a doubleheader at Griffith Stadium.

Moncewicz was inserted as a late-inning replacement for shortstop Wally Gerber, and was struck out by Washington hurler Firpo Marberry in his only at-bat.

[20] Moncewicz's third and final major league appearance came on July 2 in the opener of a doubleheader at Shibe Park against Connie Mack's Philadelphia A's, a team that boasted several future Hall of Famers in the day's lineup, including Ty Cobb, Jimmie Foxx and Lefty Grove.

[1] During World War II, Moncewicz was a lieutenant commander in the United States Navy, and served aboard the aircraft carrier USS Antietam in the South Pacific.

[1] Following his service, Moncewicz was named to the post of state comptroller by Massachusetts Governor Maurice J. Tobin in 1946, serving continuously until 1958, when he retired for health reasons.