His 255 homers as a Cardinal placed him behind only teammate Stan Musial's 475 at the time; he held the team record for a right-handed hitter from 1962 until Albert Pujols passed him in 2007. A five-time Gold Glove Award winner, he also led the NL in double plays five times and retired among the all-time leaders in games (6th, 1,785), assists (6th, 3,652) and double plays (3rd, 355) at third base. The Cardinals inducted Boyer into the team Hall of Fame in 2014.
Born in Liberty, Missouri, Boyer grew up in Alba, Missouri as the fifth of 14 children, and third oldest son, of marble cutter Chester Vern Boyer (1903–1981) and his wife, the former Mabel Agnes Means (1907–1971), including sons Cloyd (born 1927), Wayne (born 1929), Ken, Lynn (born c.1935), Clete (1937–2007), Ronnie (born 1944) and Lenny (1946–2013) and daughters Juanita Woodmansee, Leila, Dolores Webb, Pansy Schell, Shirley Lockhart, Bobbi McNary and Marcy Layton. He attended Alba High School. All seven boys played professional baseball, with two of his brothers also reaching the major leagues: older brother Cloyd was a pitcher for the Cardinals in the early 1950s, and younger brother Clete became a third baseman for the New York Yankees and Atlanta Braves.
Boyer made his major league debut on April 12, 1955 in a 14–4 road loss to the Chicago Cubs, hitting a two-run homer in the eighth inning off Paul Minner as his first hit. He batted .264 with 62 RBI as a rookie before earning the first of seven NL All-Star selections in 1956, starting at third base and batting cleanup for the National League; he finished the 1956 season with a .306 batting average, 26 home runs and 98 RBI, and led NL third basemen in assists (309) and double plays (37). He was shifted to center field in 1957 to allow rookie Eddie Kasko to break in at third, and led all NL outfielders in fielding percentage, but returned to third base in 1958, winning the first of four consecutive Gold Gloves and again collecting 90 RBI while batting .307 and scoring 100 runs for the first time. That year he also became the Cardinals' regular cleanup hitter, a role he would hold regularly for the remainder of his time with the club. His 41 double plays in 1958 equalled the second-highest total in NL history to that point, and fell just two short of Hank Thompson's 1950 league mark; he also led the league in putouts (156).
After becoming the team captain in 1959, Boyer compiled a 29-game hitting streak from August 10 to September 12 of that year, during which he batted .350 with eight home runs and 23 RBI; it was the longest hitting streak in the major leagues since Musial's 30-game run in 1950. Boyer finished 10th in the MVP voting that season after batting .309 with 28 home runs and 94 RBI, and began a run of six consecutive All-Star selections, starting the second of the two 1959 games; he again led the NL with 32 double plays. In 1960–61 Boyer led the Cardinals in batting average (.304 and .329), home runs (32 and 24), runs (95 and 109), RBI (97 and 95) and total bases (310 and 314), and finished 6th and 7th in the MVP voting. He led the league with 37 double plays in 1960, and with 346 assists in 1961. He was also named the NL's Player of the Month for September 1960 after batting .385. He hit for the cycle, with an additional single, in the second game of a doubleheader on September 14, 1961 against the Cubs, becoming the first player in history to complete the cycle with a walk-off home run in the bottom of the 11th inning for a 6–5 victory; his RBI double in the 9th inning had tied the game. In that game he also joined Musial as the only Cardinals to hit two walk-off home runs in a season in two different years; Boyer also had two walk-off homers in 1958 (May 31 and June 11; he was the fourth Cardinal to hit two extra-inning walk-off homers in a season, with both leading off the bottom of the 12th inning) and a previous one in 1961 on August 8. On September 19, 1962, Boyer broke Rogers Hornsby's team record for home runs by a right-handed hitter with his 194th career round-tripper, a 2-run shot off Billy O'Dell in the first inning of a 7–4 loss to the San Francisco Giants. He finished the season with 98 RBI, equaling his career best to that point, and started both All-Star games, also leading the league in double plays for the last time with 34. On June 7, 1963, Boyer became the second Cardinal to hit 200 career homers, connecting off Al Jackson in the 4th inning of a 3–2 road loss to the New York Mets. Again named to the All-Star starting lineup, he increased his RBI total to 111 that year, and also picked up his fifth Gold Glove.
Boyer had his best season in 1964, keeping the Cardinals alive for much of the season as he batted .350 in May and .342 in July, and starting for the NL in his last All-Star appearance. On June 16, he became the 19th player in major league history to hit for the cycle twice, and the seventh to hit for a natural cycle, in a 7–1 road victory against the Houston Colt .45s. Boyer's productivity early in the season kept the team in contention, although they were still only 54–51 and tied for fifth place on August 4; they fell 11 games out of first place by August 23, but mounted one of the great comebacks in history, overtaking the Philadelphia Phillies in the final weeks to win the NL pennant by a single game; Boyer batted .400 in five September games against the Phillies. He enjoyed his career highlight against the New York Yankees in the 1964 World Series, hitting a grand slam in Game 4 off pitcher Al Downing to give the Cardinals a 4–3 victory. His brother Clete, playing in his fifth consecutive Series with the Yankees, later conceded that he was privately thrilled for his brother because it was Ken's first Series. Then, in the decisive Game 7, he collected three hits (including a double and a home run), and scored three runs as St. Louis clinched the World Championship 7–5, their first title since 1946. Clete also homered in that game, the only time in World Series history that brothers have homered in the same game. Boyer earned National League MVP honors after hitting .295 with 24 home runs and leading the league with 119 RBI, becoming the first NL third baseman to do so since Heinie Zimmerman in 1917; he was also honored as The Sporting News Major League Player of the Year, and received the Lou Gehrig Memorial Award for character and integrity. It was also his seventh consecutive season of 90 or more RBI, tying Pie Traynor's major league record for third basemen. Boyer hit exactly 24 home runs in each of 4 consecutive years (1961–1964) (32 homers in 1960 and 13 homers in 1965) to set a record for most consecutive years with the same home run total and at least 20 home runs; the record was tied by Fred Lynn of the California Angels and Baltimore Orioles (23 each year from 1984 to 1987). On July 10, 1965, Boyer hit his 250th home run off Dick Ellsworth in the 9th inning of a 5–3 road loss to the Cubs, and on September 28 he became the fifth Cardinal to drive in 1,000 runs, in the 9th inning of a 9–1 road win against the Giants.
After 11 years with the Cardinals, Boyer began to suffer back problems in 1965, but still led the league in fielding percentage (.968) for the only time in his career; after batting just .260 with 13 homers and 75 RBI, in October 1965 he was traded to the New York Mets for Al Jackson and third baseman Charley Smith. With the downtrodden Mets, he was stuck on a losing team but managed to achieve several more career milestones. On May 13, 1966, he scored his 1,000th run in a 5–4 17-inning loss to the Giants; he ended the year batting .266 with 14 home runs and 61 RBI. On May 10, 1967, he collected his 2,000th career hit, a single off Milt Pappas in the 4th inning of a 7–4 loss to the Cincinnati Reds, and on May 20 he hit his 300th career double off Nelson Briles in an 11–9 loss to the Cardinals. On July 27, with Boyer batting .235, the Mets traded him to the Chicago White Sox along with second baseman Sandy Alomar, in exchange for third baseman Bill Southworth, whose career ended after he spent the remainder of the year in the minors, and catcher J. C. Martin. Boyer hit .261 over the rest of the season, but the White Sox released him on May 2, 1968 after he batted only .125 in 10 games. He signed with the Los Angeles Dodgers on May 10, hitting .271 in his return to the NL, and appeared in his 2,000th game on September 7 in a 4–2 loss at Cincinnati. Boyer returned to the Dodgers in 1969, but was used almost exclusively as a pinch hitter; he appeared in his last game on August 9, 1969, a 4–0 loss to the Cubs, grounding out as a pinch hitter in the 9th inning. After batting just .206 in 25 games that season, Boyer felt that his playing career was over and desired to become a coach; the Dodgers encouraged him to return as a player for the 1970 season, feeling that young players would be more likely to listen to him as a veteran player than as a coach, but Boyer chose to retire. In his 15-year career, Boyer was a .287 hitter with 2,143 hits, 282 home runs and 1,141 RBI, 1,104 runs scored, 318 doubles, 68 triples and 105 stolen bases in 2,034 games played; he also batted .348 with two home runs in his ten All-Star Game appearances. His career slugging average of .462 ranked third among players with at least 1,000 games at third base, behind Eddie Mathews (.509) and Ron Santo (then at .478), and among NL players he trailed only Mathews in assists and double plays at third base. Upon Clete's retirement in 1971, the Boyers' 444 career home runs (282 by Ken, 162 by Clete) were the fourth most in major league history by two brothers, behind Hank and Tommie Aaron (768) and the separate pairings of Joe DiMaggio with his brothers Vince (486) and Dom (448). Boyer's 12 career walk-off hits for the Cardinals remain a record for any player since 1950, equaled only by Lou Brock and Albert Pujols. On April 28, 2007, Pujols broke his Cardinals record for right-handed hitters with his 256th career home run, in an 8–1 loss to the Cubs; he had tied the mark six days earlier with a 3-run homer in the 12th inning at Chicago.
Boyer became eligible for election to the Baseball Hall of Fame in 1975, at a time when the Baseball Writers' Association of America (BBWAA) had only elected one third baseman in thirty elections (Traynor in 1948); despite his 500 home runs, Mathews had received just 32% of the vote one year earlier. Boyer fared no better in the voting than most other stars at the position, and received less than 5% of the vote every year before being dropped from consideration after the 1979 vote. After several years of complaints about overlooked candidates, Boyer was one of three players restored to the ballot in 1985, along with Cardinals outfielder Curt Flood and fellow third baseman Ron Santo – who had himself received less than 4% of the vote in his only year on the ballot in 1980. For four years, Boyer outpaced Santo in the voting, peaking at 25.5% of the vote in 1988; but popular support for Santo grew afterward, and he outdrew Boyer every year afterward until Boyer's eligibility expired after the 1994 vote. Santo received 43% of the vote in his final year in 1998. Boyer has since been a candidate on the final Veterans Committee ballot in 2003, 2005, 2007, and 2012; Santo's induction in 2012 has led to suggestions that Boyer's chances of election have increased. Boyer was inducted into the St. Louis Sports Hall of Fame in 2012. In 2014, the Cardinals announced Boyer among 22 former players and personnel to be inducted into the St. Louis Cardinals Hall of Fame Museum for the inaugural class of 2014.
Ken Boyer married Kathleen Oliver in April 1952. The couple had four children – Susie, David (born December 28, 1955), Danny, and Janie – but eventually divorced; David was drafted by the Cardinals in 1974 and played in their farm system until 1978. Boyer died from cancer in St. Louis, Missouri on September 7, 1982 at the age of 51; he had undergone laetrile treatments in Mexico in an attempt to fight the disease. He was survived by twelve of his thirteen siblings, and by his four children. He was buried in Friends Cemetery in Purcell, Missouri. His #14, which he wore throughout his career with the Cardinals, was retired by the team in 1984.