In 1918, injuries—including a groin pull, a spiked thumb, and a sore shoulder—hampered Hornsby, and he slumped to .281, the worst full-season performance of his major-league career. The season was cut short by World War I, and Hornsby was drafted to go to Wilmington, Delaware, to work in the naval shipyards and play ball for military teams. There, he married Sarah Martin. The war ended, and over the winter Hornsby and his bride traveled around Texas and set up a string of automobile dealerships, capitalizing on his sudden fame. In 1919 Hornsby was groomed for second base but played all four infield positions and rebounded to hit.318, almost winning the batting championship.
The next year, Hornsby was installed at second base, the position that Cardinals management decided he was best suited to play. Settled there for the entire year, Hornsby had a breakout season, winning the batting championship with a robust .370 average and leading the league in hits, runs batted in, doubles, and slugging percentage.
In his prime, Hornsby piled up offensive numbers that have never been equaled. The 1920 season was the first of six consecutive seasons, and seven overall, in which Hornsby won the National League batting championship. His accomplishments over the next five-year stretch, 1921 through 1925, could hardly have been imagined a few years earlier, when he was a scrawny kid struggling to hit. It helped that baseball had instituted a new, livelier ball, but even in that context his achievements were astounding. No one before or since has averaged over .400 for a five-year period, even Cobb, but Hornsby hit .397, .401, .384, .424. and .403. He didn't just hit for average. He compiled high on-base percentages with many walks and increased his power output. In 1922, he won the Triple Crown (batting average, home runs, and RBIs) and set a new National League record with forty-two home runs. He drove in 152 runs, and led the league with 250 hits, forty-six doubles, 141 runs scored, and a slugging mark of .722 along with a .459 on-base percentage. It was almost certainly the best offensive season in league history—at least until Barry Bonds' performances in 2001 and 2002—and was surpassed only by the top seasons of his American League contemporary Babe Ruth.
Everything seemed to have fallen in place for Hornsby. He was the proud father of a little boy, Rogers Jr., and the top player in the National League. But Hornsby's love of gambling soon led to some off-the-field troubles. One day in 1922, he met a married woman named Jeannette Pennington Hine at a dog track. After the 1922 season, both spouses found out about the affair. Hornsby's wife Sarah took their son and left him to live with her mother. In 1923, Hornsby had to overcome a serious knee injury, legal problems stemming from his extramarital affair including a costly divorce settlement, and his mother's illness. At the end of the season, he was fined and suspended for refusing to play. Still, he hit.384 and qualified for the batting title despite playing in only 107 games. In February 1924, with all legal matters settled, Hornsby married Pennington.
In 1924 Hornsby compiled the highest batting average by any player in the post-1900 era—.424. That year, he played in 143 games and hustled out every ball he hit, hitting safely in 119 games. Hornsby's batting feat won him national attention and for that year he even eclipsed the fame of Babe Ruth, America's most popular athlete. Hornsby led the league in hits, doubles, runs, and slugging percentage, and also in on-base percentage and walks. Incredibly, he failed to win the league's Most Valuable Player award, which was bestowed on Brooklyn Dodgers pitcher Dazzy Vance.