Cal Hubbard was born in Keytesville, Missouri to parents Robert P. and Sarah "Sallie" (Ford) Hubbard. He grew up in modest means as the son of a small family farmer. Cal graduated from Keytesville High School, but because the school had no football team he also attended one year at Glasgow High School in nearby Glasgow, Missouri, which did offer football. Already tall and weighing 200 pounds as a 14-year old, Hubbard displayed natural athletic gifts. He aspired to attend the United States Military Academy at West Point, New York; however a physical discovered he had flat feet, eliminating him from eligibility. Instead Hubbard chose to attend a college or university that offered football, selecting Chillicothe Business College in Chillicothe, Missouri while also continuing to work around his family farm.
A chance meeting in 1922 with Bo McMillin, the new football coach at Centenary College in Shreveport, Louisiana, led Hubbard to enroll and play football there from 1922 to 1924. When McMillin moved on to suburban Pittsburgh's Geneva College, Hubbard followed him and played there in 1926 (after a year of ineligibility for switching schools in 1925). Noted for remarkable speed for a player of his size (6' 4", 250 lb or 1.93 m, 115 kg), he starred as a tackle and end, playing off the line in a style similar to that of a modern linebacker. Hubbard completed his college education in 1927, earning a Bachhelor of Arts degree from Geneva College.
Hubbard moved on to the National Football League in 1927, signing with the New York Giants for a salary of $150 per game. Playing alongside Steve Owen his rookie year, he helped the Giants defense allow opponents to score just twenty total points all season as they won the league championship. For his efforts Hubbard won all-league honors by the press the following year. But with a lifelong dislike for big cities, he didn't feel comfortable in New York and a 1928 road game in Green Bay led him to request a trade to the Packers, threatening to retire otherwise.
Under Packers coach Curly Lambeau, Hubbard and the team won the NFL title in each of his first three years there (1929–1931). The NFL named its first official All-League team in 1931 with Cal Hubbard being one of that inaugural list. He was chosen for the honor again in 1932 and 1933. Hubbard stepped away from professional football following the 1933 season, taking a job as the line coach at Texas A&M in 1934. However he was persuaded to return to play after that one year on the sidelines, returning to Green Bay in 1935. The Giants wooed him back to start 1936 with them, but he played only six games the entire season, five for the Giants and a final game for the Pittsburgh Pirates, the future Steelers. Cal Hubbard returned to football coaching briefly, serving as head coach of his alma mater Geneva College in 1941 and 1942. He was among the initial class of inductees to the Pro Football Hall of Fame in 1963. In a 1969 poll by the Hall of Fame committee, Hubbard was voted the NFL's greatest tackle all-time.
Even while Hubbard's football career was going full-force he began to focus on a second career in baseball officiating. From 1928 onward he spent his football off-season umpiring in minor league baseball. By 1936 Hubbard had been called up to the major leagues, umpiring in the American League from 1936 to 1951. Soon recognized as one of the game's best officials, he was selected to work in the 1938 World Series, followed by Series appearances in 1942, 1946 & 1949. In addition, he umpired in the All-Star Game in 1939, 1944 & 1949, behind the plate for half of the 1939 & 1944 midsummer classics. As an umpire Hubbard found the then-common practice of officials moving to different positions on the field during a game to be confusing and hampered accuracy when making calls. Applying his football experience to baseball, he devised a system where each official had clearly defined duties and also added an additional official to the crew. This was the foundation on which Major League Baseball established new officiating standards in 1952.
While hunting during the 1951 off-season a ricocheting pellet from a friends shotgun blast accidentally struck Hubbard in the right eye. The damage was extensive enough to force his retirement from baseball officiating. However the American League soon hired him as an assistant supervisor for league officiating crews, and in 1954 he became the top supervisor, a position he would hold until retiring for good in 1969. In recognition of his contributions to the game as an umpire and supervisor, Cal Hubbard was elected to the Baseball Hall of Fame in 1976, only the fifth umpire to be so honored up to that time.
Never a fan of big cities, Cal Hubbard first settled in Milan, Missouri in 1944. He left in 1945 but moved back in 1948 for good. Milan was a small town much like his native Keytesville, which is about fifty miles to the south on Missouri Route 5. A lifelong avid fisherman and hunter, the rural areas of Sullivan and neighboring counties offered ample opportunity. Hubbard stayed involved in community affairs, especially children's sports, as much as his schedule would allow. In honor of his efforts the football field at Milan High School is named Cal Hubbard Field. Cal Hubbard died due to cancer October 17, 1977 at age 76 in St. Petersburg, Florida. He is buried in the Oakwood Cemetery, Milan, Missouri. He was survived by wife Mildred and two sons, Dr. Robert Hubbard, and William "Bill" Hubbard.