In his second season (1925) Simmons led the AL with 253 hits with a .387 batting average, 24 home runs and 129 runs batted in (RBI). He scored 122 runs, hit 43 doubles, and finished with a .599 slugging percentage. He earned the second-most votes for the league's Most Valuable Player Award. In the following three seasons, he hit .341, .392 and .351 and drove in 109, 108 and 107 runs in those respective years, while finishing fifth in 1926 MVP voting and fourth in 1927.
Simmons led the A's to the AL pennant in 1929 as Philadelphia went 104–46, finishing 18 games ahead of the New York Yankees. The A's went on to defeat the Chicago Cubs in five games to win the World Series. That season he hit .365 with 34 home runs and led the AL with 157 RBI. He also scored 114 runs, had 212 hits with 41 doubles and a .642 slugging percentage. In his first World Series Simmons batted .300 with 2 home runs, 5 RBIs and scored 6 runs.
Simmons' best year as a player was in 1930, when he won his first of successive batting titles, hitting .381 with 36 home runs, 211 hits, 41 doubles and 16 triples. He had a slugging percentage of .708, drove in 165 runs and scored 152 runs in 138 games. The A's won the AL pennant again, going 102–52, and defeated the St. Louis Cardinals to win back-to-back World Series titles. In that World Series, Simmons batted .364 with 2 home runs, 4 RBI with a .727 slugging percentage.
In 1931 the A's won their third straight AL pennant by 13.5 games over the Yankees, going 107–45. Simmons had another great season, winning his second batting title. He hit .390 with 22 home runs, 128 RBI, 100 runs scored, 200 hits, 37 doubles, 13 triples and a .641 slugging percentage while playing in only 128 games. He finished third in AL MVP voting behind the MVP (his teammate Lefty Grove) and the Yankees' Lou Gehrig. The A's were upset in their quest for a third World Series title, losing the World Series in seven games to the Cardinals. Still Simmons hit .333 with 2 home runs and 8 RBI in the series.
In a final season with Philadelphia, Simmons led the AL with 216 hits. He batted .322, with 35 home runs, 151 RBIs and 144 runs scored in 1932. With Connie Mack suffering financially, Simmons was traded to the Chicago White Sox for the 1933 season. Simmons finished his Philadelphia Athletics tenure with a .356 batting average, 209 home runs, 1,178 RBI and 969 runs scored in 1,290 games. He drove in 100+ runs in all nine seasons and scored 100 or more runs in five seasons. In 3 World Series appearances for the A's he hit .333 with 6 home runs, 17 RBI and 15 runs scored in 18 games.
During his first season in Chicago, Simmons batted .331 with 14 home runs, 119 RBI and 200 hits. In 1934 he batted .344 with 18 home runs, 104 RBI, 102 runs scored and 192 hits in 138 games. After a disappointing final season with the White Sox which saw Simmons bat just. 267 with 16 home runs and 79 RBI in 128 game (first time in his 11-year career he did not reach .300+ & 100 RBI) he rebounded by hitting .327 with 13 home runs, 112 RBI and 96 runs scored in 1936 for the Detroit Tigers.
In 1937 he struggled again, this time with the Washington Senators, batting just .279 with 8 home runs and 84 RBIs in 103 games. He rebounded with a stellar season in 1938, batting .302 with 21 home runs and 95 RBI in just 125 games for Washington in 1938. His 21 home runs that year gave Simmons the distinction of being the first player to hit 20 home runs in a year for the Senators.
A "bucketfoot" hitter (his nickname was "Bucketfoot Al") who strode toward third base when hitting, Simmons hit 307 career home runs. He compiled more hits than any right-handed batter in AL history until surpassed by Al Kaline. A deadly clutch hitter and a favorite of manager Connie Mack, Simmons won batting titles in 1930 and 1931 to help the A's to consecutive pennants. He recorded a .300 batting average and 100 or more RBI in his first eleven major league seasons. Simmons accumulated 2,000 hits in 1,390 games, which remains the shortest number of games needed to attain that mark in major league history.
He played from 1924 through 1941, then appeared in 1943–44. He earned a lifetime batting average of .334. He hit .340 or better in eight different seasons, with four seasons of better than .380. He also compiled 200 hits or better in a season six times, with five of those being consecutive (1929–33), and 199 and 192 hits in 1926 and 1934 while only playing 147 and 138 games in those years. He also hit for power, finishing in the top six in AL in home runs for seven consecutive seasons (1925–32). He also hit .329 with 6 home runs, 17 RBI, 15 runs scored and .658 slugging percentage in 19 World Series games.
After his playing days ended, Simmons served as a coach for Mack's Athletics (1945–49) and the Cleveland Indians (1950). He was elected to the Wisconsin Athletic Hall of Fame in 1951 and was elected into the National Polish-American Sports Hall of Fame in 1975. Simmons died of a heart attack in Milwaukee at age 54. In a 1976 Esquire magazine article, sportswriter Harry Stein published an "All Time All-Star Argument Starter," consisting of five ethnic baseball teams. Simmons was the right fielder on Stein's Polish team. He was buried at St. Adalbert's Cemetery in Milwaukee. In 1999, he ranked number 43 on The Sporting News' list of the 100 Greatest Baseball Players, and was a nominee for the Major League Baseball All-Century Team.