He appeared in 1,406 games over a 12-year career in Major League Baseball as a first baseman and left fielder for the New York Yankees, Kansas City Athletics, Baltimore Orioles, California Angels, San Francisco Giants and Boston Red Sox between 1956 and 1968.

Recalled from Triple-A Denver in June, Siebern appeared in 54 games, starting 39 in left field, second-most to Elston Howard's 50, but, hampered by injury,[2] he hit only .204 with four home runs in 162 at bats.

With all World Series games then played in the afternoon, and home plate in autumnal shadows, left field was a notoriously treacherous position at Yankee Stadium: outfielders often were forced to track fly balls while staring into a blinding sun.

In a seven-player trade, he was dealt to the second-division Kansas City Athletics, along with veteran World Series heroes Don Larsen and Hank Bauer and future New York Mets legend Marv Throneberry, in exchange for 25-year-old outfielder Roger Maris and two others, first baseman Kent Hadley and shortstop Joe DeMaestri.

He hit 78 home runs in a Kansas City uniform, and was selected to three American League All-Star squads as a first baseman, where he moved permanently in June 1961.

In his finest offensive season, 1962, Siebern led the American League in games played (162) and finished second (behind Hall of Famer Harmon Killebrew) in runs batted in, third in on-base plus slugging percentage (OPS) (.907), and seventh in MVP balloting.

Siebern moved into the lineup of the first-division and pennant-contending Orioles as their regular first baseman in 1964, starting in 138 of the club's 162 games and being selected to his final All-Star berth.

In 1965, Siebern began to split first-base duty with 23-year-old former Orioles' left fielder Boog Powell; he started 67 games and batted .256 with eight home runs.

With Powell becoming entrenched as Baltimore's regular first-sacker, Siebern was earmarked for trade and he became an important footnote to perhaps the most momentous transaction in Oriole history.

On July 15, the Giants sent him back to the American League, selling his contract to the Junior Circuit's surprise pennant contender, the "Impossible Dream" Red Sox.

[6] After the Red Sox prevailed in the pennant race by one game, Siebern participated in his third and last World Series, the 1967 Fall Classic against the St. Louis Cardinals.

In the seventh inning of Game 1, he pinch hit for Red Sox catcher Russ Gibson, but was left at home plate when Reggie Smith was caught stealing to end the frame.

Siebern played basketball at Southwest Missouri State alongside his future New York and Kansas City baseball teammate Jerry Lumpe on a team that won two NAIA Championships in 1952 and 1953.