Horlen was signed by the Chicago White Sox in. That year he pitched for the Lincoln Chiefs. The next season he pitched in Class A for the Charleston White Sox, and was 7-5 with a 2.93 ERA. He began 1961 pitching for the AAA San Diego Padres, for whom he was 12-9 with a 2.51 ERA.
Major league career
Chicago White Sox (1961–71)
He made his Major League debut against the Minnesota Twins in the second game of a September 4, doubleheader. He won the game in relief while wearing a numberless uniform —- as the only available road uniform did not have a number. Horlen pitched as a spot starter in his first two full seasons with the White Sox. In 1963, he returned to the minors to pitch four games for the AAA Indianapolis Indians, going 3-0 with a 1.74 ERA. In he earned a spot in the starting rotation, posting a 13–9 record and setting career bests in earned run average and strikeouts. He also led the majors by allowing only 6.07 hits per 9 innings, bettering Sandy Koufax's National League-leading 6.22. In the next 42 years, only 8 right-handed pitchers bettered that ratio in a season. He also led the AL in Walks + Hits per IP . That year his White Sox battled the New York Yankees and Baltimore Orioles for the pennant, but finished second, one game behind the Yankees and one game ahead of the Orioles. In 1965 he was 2nd in the league in shutouts, and was 3rd in walks/9 IP. In 1966 he led the league in wild pitches, was 6th in hit batsmen, and was 2nd in ERA. Horlen's best season was in ; he finished 19–7 and led American League pitchers with a 2.06 ERA and 6 shutouts, was 2nd in W-L percentage, 4th in wins, complete games, and walks/9 IP, and 7th in innings pitched. He also led the AL in Walks + Hits per IP . He was named to the American League All-Star team for the only time in his career, but did not pitch in the game. The highlight of Horlen's season was a clutch performance on September 10 as the White Sox were involved in a four-way pennant race with the Twins, Boston Red Sox, and Detroit Tigers; he no-hit the Tigers in the first game of a doubleheader at Comiskey Park. Not until the Tigers' Jack Morris no-hit the White Sox in would another no-hitter be pitched in a White Sox home game, and the next no-hitter by a White Sox in a White Sox home game wouldn't be pitched until, by Mark Buehrle at U.S. Cellular Field. Horlen recorded victories in his next three starts, the next one coming five days later against the Twins. However, on September 27, which would be known by White Sox fans as "Black Wednesday", the lowly Kansas City Athletics swept a doubleheader from the White Sox and effectively eliminated Eddie Stanky's "Hitless Wonders" from pennant contention. Horlen lost the second game, with 21-year-old Catfish Hunter shutting out the White Sox 4–0. The two games were the last played by the Athletics in Kansas City; they moved to Oakland for the start of the season. The White Sox finished fourth, three games behind the Red Sox who, after finishing next to last in, won the pennant on the final day, finishing one game ahead of the Twins and Tigers. Horlen finished runner-up to Jim Lonborg, the star of the Red Sox staff, in the American League Cy Young Award balloting, and 4th in MVP voting, won by another Red Sox, Carl Yastrzemski. In 1968 he led the AL in hit batsmen. In 1970 he was 5th in walks/9 IP. In spring training of, two weeks after voting unanimously in favor of a strike, the White Sox released Horlen, who had been the Sox’ player representative.
Oakland Athletics (1972)
He later signed with Oakland, and pitched mostly in relief as the Athletics won the World Series — the first World Series title for the franchise since the Philadelphia Athletics in 1930.
