The Hornsby family came from Wales to colonial Virginia in the early 18th century, and about a century later moved to west Texas, to a settlement near Austin that became known as Hornsby's Bend. Ed Hornsby married Mary Dallas Rogers, from nearby Rogers Hill, in 1882. They had four children before moving to a homestead near Abilene. There, Rogers Hornsby—named after his mother—was born on April 27, 1896. His father died two years later of unknown causes. His mother took the children to live with her parents on a farm near Austin, then later moved to Fort Worth, a booming meatpacking town. In Fort Worth, Rogers began playing baseball in earnest, and by the time he was nine was the star of a local team. Mary Hornsby sewed his team's blue flannel uniforms, and the boys traveled to their games by trolley. By age ten, when he went to work after school and during the summer for the Swift and Company meatpacking plant, he substituted on teams of stockyard and packing plant workers.
Young Hornsby was slight, smooth on defense at any position, and cocky, but not much of a hitter. But he was determined to be a professional baseball player. At age fifteen, he was playing on the North Side Athletics in the Fort Worth adult league, and on other teams, including the semipro Granbury club, where he made two dollars per game plus room and board and rail fare. Granbury manager H.L. Warlick remembered praising Hornsby after one game for making all the plays at second base, and recalled Hornsby replying: "Yeah, and there are eight other positions I can play just as good," according to Charles Alexander's biography, Rogers Hornsby. The next summer, Hornsby and a friend took a train to Dallas, answering a newspaper ad looking for players under eighteen to play with the touring women's team, the Boston Bloomer Girls. Donning wigs and bloomers, Hornsby and his friend played as female impersonators.
In high school, Hornsby played football and baseball, but dropped out after two years to help support his mother
by working as an office boy at Swift and Company. In the spring of 1914, Hornsby had grown to his adult height of nearly six feet but still weighed only 135 pounds. His older brother Everett, who had played seven years in the Class B Texas League and one season at Class A with Kansas City, was on the Dallas Steers and got his little brother a tryout. He signed a contract but never played and was released after two weeks. He tried out with a new team in the Class D Texas-Oklahoma League, the lowest level of the minors. He made the team, located in the small town of Hugo, and played for $75 a month as the regular shortstop. But the franchise folded and Hornsby's contract was sold to Denison. There, he hit .232 and made forty-five errors in 113 games. "Won't somebody teach me how to hit?" he pleaded to teammate Herb Hunter, according to Alexander.
After another season at Denison, where he boosted his average to .277 but still made fifty-eight errors in 119 games, Hornsby was called up at the end of 1915 to St. Louis, a struggling National League club. Rosters in the major leagues had been severely depleted by players defecting to the upstart Federal League. Hornsby, whose record to that point hardly merited such a rapid promotion, arrived in St. Louis as a callow, wholesome young man, and he frequently got lost in the unfamiliar big city. He played in his first game on September 10, a week after joining the club, and was hitless in two at-bats. In all, he batted .246, appearing in eighteen games at shortstop and making many mistakes in the field.
After the season, when the manager told Hornsby he wanted to "farm him out for a year"—send him down to the minors for more seasoning—Hornsby misunderstood him and decided he would spend the off-season on his uncle's farm near Austin. He worked hard, ate steak and fried chicken, drank a lot of fresh milk, and slept twelve hours a night. He put on thirty-five pounds. A bulked-up Hornsby returned to spring training a much more impressive hitter. He stood far back in the box and away from home plate, used an open stance, and rifled line drives to all fields. Confident, hustling, and eager, Hornsby forced his way onto the team and into the regular lineup and played every infield position, finishing the season primarily as a third baseman and hitting .313. By the end of the season, several other National League clubs were trying to get Hornsby in a trade.
In 1917, Hornsby hit .327 and led the league with 253 total bases, a .484 slugging percentage and seventeen triples, in an era when triples were the prime measure of a power hitter. His defense was nothing to brag about, though. Playing the entire year at shortstop, he made fifty-two errors, third-highest in the league.