"Harry the Hat" got his nickname from his habit during at-bats of continually adjusting his cap between pitches—there were no batting helmets in his day. His batting title came in 1947, when he hit .363 in a season during which he was traded from his original team, the St. Louis Cardinals, to the Philadelphia Phillies. The previous year he was one of the stars of the Cardinals’ 1946 World Series championship team. In the decisive seventh game against the Boston Red Sox, with Enos Slaughter on first base, Harry doubled to left center and Slaughter, running on the pitch and taking advantage of a slow relay from the Red Sox' Johnny Pesky, scored from first base with the winning run. He knocked in six runs during that Series, and batted .412. Harry lacked his brother Dixie's power—he hit only ten home runs in all or parts of 11 seasons in the National League—but he compiled a .296 lifetime batting average with the Cards, Phils, Chicago Cubs and Cincinnati Reds and was to be famed throughout his coaching and managing career as a batting tutor. Harry and Dixie are the only brothers in MLB history to win batting titles, Dixie won the National League batting title with a .357 average in 1944 while playing for the Dodgers, while Harry accomplished the historical feat in 1947 after a trade from the Cardinals to the Philadelphia Phillies winning the National League batting title with a .363 average.
After prepping as a skipper in the Cardinals’ minor league system beginning in 1951, Walker was called up from Rochester in the AAA International League on May 28, 1955, to replace Eddie Stanky as Cardinals’ manager. However, the change backfired: the Cards plummeted two places in the standings under Walker, losing 67 of 118 games. Harry was replaced by Fred Hutchinson at the end of the 1955 season, and it would be another decade before he would again manage in the majors.
During that exile, he returned to the Cardinal farm system to manage (1956–58; 1963–64), and served four years (1959–62) as a St. Louis coach. After piloting the Jacksonville Suns to the 1964 International League pennant, Walker was hired by the Pittsburgh Pirates as manager, replacing Danny Murtaugh, who stepped down for health reasons. Although the Pirates did not win a pennant during Walker’s first two seasons, he made an immediate impact. His skill as a batting coach was an important factor in the transformation of the Pirates into the National League's top offensive team. The Pirates battled for the pennant until the closing days of the 1965 and 1966 seasons—each year finishing third behind the champion Los Angeles Dodgers and the runner-up San Francisco Giants. But when the 1967 Pirates—further strengthened by an off-season trade for standout shortstopMaury Wills—stumbled to a disappointing .500 mark in mid-season, Walker was let go on July 18 in favor of his predecessor, Murtaugh. Less than a week later, Walker was hired to be the organizational batting coach for the Houston Astros.
Eleven months later, on June 18, 1968, the Astros replaced skipper Grady Hatton with "Harry the Hat," still well-known from his stint as manager of the Texas League's Houston Buffaloes during the late 1950s. Featuring players like Joe Morgan, Jimmy Wynn, and Don Wilson, the Astros finished last in 1968, but their record under Walker was 49-52. In 1969, they contended for the National League West Division title before fading to finish 12 games behind the Atlanta Braves. After back-to-back 79-83 marks in 1970 and 1971, Walker was sacked August 26, 1972, in favor of Leo Durocher; with the Astros at 67-54 and in third place at the time of the firing, it was Walker’s best season in Houston. Over his managing career, he won 630 games, losing 604 (.511). After his firing, Walker returned to the Cardinals as a hitting instructor.
Walker served as the head baseball coach at the University of Alabama at Birmingham (UAB) from 1979 to 1986. He was the program's first coach. In ten seasons he compiled a record of 211-171, good for a .552 winning percentage. In 1981 the Blazers were the champions of the Sun Belt Conference's North Division in just the third year of the program's existence. The Blazers repeated as North Division champions in 1982.
Walker was profiled quite flatteringly in Jim Bouton's memoir of the 1969 season, Ball Four. In the book, Walker is seen as a knowledgeable manager who has good advice for his charges. Although many of the players complain that Walker talks too much, Bouton is careful to point out that Walker always makes a good point and has good advice. This is notable because Bouton was unafraid to show his earlier manager, Joe Schultz, in a much less flattering light. Bouton even tells a humorous story of how Walker himself would follow the advice he always gave when he played in an old timer's game. The players jokingly would yell tips that Walker always said, such as "hit the ball up the middle." Walker would then proceed to single up the middle, then break up the double play, prompting Astro third basemanDoug Rader to remark, "Son of a bitch. Every year Harry gets a hit up the middle and breaks up the double play."