Belanger was recruited by the Orioles as an amateur in 1962, and made his debut with the club on August 7, 1965. He took over as the Orioles' regular shortstop in 1967, and held the position for 15 seasons.
He was nicknamed "The Blade" because of his height (6'1"/155 cm) and weight (only 170 lbs./77 kg), and was known as a poor hitter. In his eighteen seasons in Major League Baseball, Belanger hit only 20 home runs, and had a lifetime batting average of .228. His true contribution to the team was at his position, where he earned a reputation as one of the best fielding shortstops ever. He earned the American League's Gold Glove Award at shortstop in 1969, 1971, and 1973-1978, and was named to the American League's All-Star team in 1976. His contributions led to the Orioles' World Series victory in 1970, the second in his career.
Belanger was known for his intelligence and his gentlemanly demeanor on and off the field. He was the Orioles' representative in the baseball players' union, the Major League Baseball Players' Association, for many years. Playing between star players such as Brooks Robinson and Davey Johnson, he was a favorite of many Baltimore fans, and came to symbolize the Orioles of the 1970's as a group of selfless, determined overachievers whose talents as a team were greater than their sum as individuals.
He was granted free agency in 1981, and signed with the Los Angeles Dodgers for the 1982 season, after which he retired. His eventual successor at the position for the Orioles in 1982 was Cal Ripken, Jr
Belanger contracted lung cancer in the late 1990's - perhaps because of his habit of smoking cigarettes - and died on October 6, 1998.