The Baltimore Orioles baseball franchise can trace its roots back to the original Milwaukee Brewers of the Western League (WL), beginning in 1894 when the league reorganized. The Brewers were still league members when the WL renamed itself the American League (AL) in 1900. At the end of the 1900 season, the AL removed itself from baseball's National Agreement of 1883, the formal understanding between the National League (NL) and the minor leagues, and declared itself a competing major league. During 1901, the first season the AL operated as a major league, the Brewers finished last among the league's eight teams.

In 1902, the team moved to St. Louis and became the "Browns", named after the original name of the 1880s club now known as the Cardinals. Although they usually fielded mediocre teams, they were very popular at the gate. In 1916, after years of prosperity at the gate, Robert Hedges sold the team to Phil Ball, who made a considerable effort to make the Browns competitive. However, Ball's tenure was marked by errors, including the firing of Branch Rickey, which eventually benefited the Cardinals, who shared Sportsman's Park with the Browns.

The 1944 season saw the Browns winning their only St. Louis-based American League pennant, becoming the last of the 16 teams that made up the major leagues from 1901 to 1960, to play in a World Series. In 1951, Bill Veeck purchased the Browns and introduced a series of promotions and wild antics. Veeck's efforts to drive the Cardinals out of St. Louis failed when Anheuser-Busch purchased the Cardinals. Veeck attempted to move the Browns to Milwaukee and then Baltimore, but both moves were initially blocked by other American League owners. Eventually, Veeck sold his stake to a group of Baltimore investors, and the team moved to Baltimore for the 1954 season, renaming themselves the Baltimore Orioles. The name has a rich history in Baltimore, having been used by Baltimore baseball teams since the late 19th century. The Orioles' early years in Baltimore were marked by a gradual climb to respectability, leading to their first World Series title in 1966. The Orioles enjoyed a period of sustained success from 1966 to 1983, winning three World Series titles and six American League pennants.

After the 1983 World Series win, the Orioles experienced a decline, culminating in the 1988 season where they lost the first 21 games. In 1989, the Orioles showed improvement with the "Why Not?" Orioles finishing second in the AL East. The opening of Oriole Park at Camden Yards in 1992 marked a new era, and the team returned to the playoffs in 1996 and 1997. However, the late 1990s and 2000s were characterized by losing seasons and rebuilding efforts. The Orioles saw a resurgence in the 2010s, making the playoffs in 2012, 2014, and 2016, with Buck Showalter as manager. The team struggled again towards the end of the decade, leading to a major rebuild. The 2020s have been marked by rebuilding efforts and developing young talent, aiming for future competitiveness.

Milwaukee Brewers

The modern Orioles franchise can trace its roots back to the original Milwaukee Brewers of the minor-league Western League (WL), beginning in 1894 when the league reorganized. The Brewers were still league members when the WL renamed itself the American League (AL) in 1900.

At the end of the 1900 season, the AL removed itself from baseball's National Agreement of 1883, the formal understanding between the National League (NL) and the minor leagues. Two months later, the AL declared itself a competing major league. As the baseball "war" heated up, the AL began to challenge the NL senior circuit more directly. The AL already fielded teams in Boston, Chicago, and Philadelphia, solid NL cities. It had planned to move the Brewers to St. Louis for the 1901 season, but those plans fizzled. As a result of several franchise shifts, the Brewers were one of only two WL teams that did not either fold or move (the other being the Detroit Tigers).

During , the first season that the AL operated as a major league, the Brewers finished last among the league's eight teams, with a record of . During its lone major-league season in Milwaukee, the team played at Lloyd Street Grounds, between 16th and 18th Streets in Milwaukee.

St. Louis Browns

In 1902, however, the team did move to St. Louis, where it became the "Browns", in reference to the original name of the legendary 1880s club that by 1902 was known as the Cardinals. In their first St. Louis season, the Browns finished second. Although they usually fielded terrible or mediocre teams (they had only four winning seasons from 1901 to 1922), they were very popular at the gate.

During this time, the Browns were best known for their role in the race for the 1910 American League batting title. Detroit's Ty Cobb took the last game of the season off, believing that his slight lead over Cleveland's Nap Lajoie was enough to win the title unless Lajoie had a near-perfect day at the plate. However, Cobb was one of the most despised players in baseball, and Browns catcher-manager Jack O'Connor ordered rookie third baseman Red Corriden to station himself in shallow left field. In each of his next five at bats, Lajoie bunted down the third-base line and made it to first easily. In his last at-bat, he reached base on an error – officially giving him a hitless at-bat. O'Connor and coach Harry Howell tried to bribe the official scorer, a woman, to change the call to a hit, even offering to buy her a new wardrobe. Cobb won the batting title by just a few thousandths of a point over Lajoie (though in 1978 sabermetrician Pete Palmer discovered that one game may have been counted twice in the statistics). The resulting outcry triggered an investigation by American League president Ban Johnson. At his insistence, Browns owner Robert Hedges fired O'Connor and Howell; both men were informally banned from baseball for life.

In 1916, after years of prosperity at the gate, Hedges sold the team to cold-storage magnate Phil Ball, who had owned the St. Louis Terriers of the defunct Federal League. Ball's tenure as owner saw the Browns' first period of prosperity; they were a contender for most of the early 1920s, even finishing second in 1922. He made a considerable effort to make the Browns competitive, reinvesting all profits back into the team.

Ball, however, committed several errors that dogged the franchise for years to come. His first major blunder was to fire Branch Rickey, the resident genius in the Browns' front office, in 1919 because of a conflict of egos, causing Rickey to jump to the crosstown Cardinals. In 1920 Sam Breadon, who had just purchased the Cardinals, convinced Ball to allow his team to share the Browns' home, Sportsman's Park. With the money from the sale of the Cardinals' Robison Field, Rickey began building an extensive farm system. This eventually produced a host of star players that brought the Cardinals far more drawing power than the Browns.

The 1922 Browns excited their owner by almost beating the Yankees to a pennant. The club was boasting the best players in franchise history, including George Sisler, and an outfield trio – Ken Williams, Baby Doll Jacobson, and Jack Tobin – who batted .300 or better in 1919–1923 and in 1925. In 1922, Williams became the first player in major league history to hit 30 home runs and steal 30 bases in a season, something that was not done again in the majors until 1956.

Ball confidently predicted that there would be a World Series in Sportsman's Park by 1926. In anticipation, he increased the capacity of his ballpark from 18,000 to 30,000. There was indeed a World Series in Sportsman's Park in 1926, but it was the Cardinals, not the Browns, who played in it, upsetting the Yankees. St. Louis had been considered a "Browns' town" until then; after 1926, the Cardinals dominated St. Louis baseball, while still technically tenants of the Browns. Meanwhile, the Browns rapidly fell into the cellar. They had only two winning records from 1927 to 1943, including a 43–111 mark in 1939 that is still the worst in franchise history.

Ball died in 1933. His estate ran the team for three years until Rickey helped broker a sale to investment banker Donald Lee Barnes, whose son-in-law, Bill DeWitt, was the team's general manager. To help finance the purchase, Barnes sold 20,000 shares of stock to the public at $5 a share, an unusual practice for a sports franchise.

War Era

1944

In 1944, during World War II, the Browns won their only St. Louis-based American League pennant, becoming the last of the 16 teams that made up the major leagues from 1901 to 1960 to play in a World Series. By comparison, each of the other seven American League teams had appeared in at least three World Series, and won it at least once. Some critics called it a fluke, as most major league stars voluntarily joined or were drafted into the military; however, many of the Browns' best players were classified 4-F (unfit for military service). They faced their local rivals, the more successful Cardinals, in the 1944 World Series, the last World Series played entirely in one stadium until 2020, and lost 4 games to 2. After the season, Barnes sold the Browns to businessman Richard Muckerman, who cared more about improving Sportsman's Park than getting better players.

1945–1946 In 1945, the Browns posted an 81–75 record and fell to third place, six games out, again with less than top-ranked talent. The 1945 season may be best remembered for the Browns' signing of utility outfielder Pete Gray, the only one-armed major league position player in history. They tumbled to seventh in 1946. They never finished with a winning record again while in St. Louis, partly because Muckerman was unwilling to improve the on-field product. Muckerman sold the team to DeWitt in 1949, but DeWitt was unable to reverse the slide.

Bill Veeck's Browns In 1951, Bill Veeck, the colorful former owner of the Cleveland Indians, purchased the Browns. In St. Louis, he extended the promotions and wild antics that had made him famous and loved by many and loathed by many others. His most notorious stunt in St. Louis came on August 19, 1951, when he instructed Browns manager, Zack Taylor to send Eddie Gaedel, a 3-foot 7 inch, 65-pound midget, to bat as a pinch hitter. When Gaedel stepped to the plate he was wearing a Browns uniform with the number , and little slippers turned up at the end like elf's shoes. With no strike zone to speak of, Gaedel walked on four straight pitches. The stunt infuriated American League President Will Harridge, who voided Gaedel's contract the next day. In another Veeck stunt, the Browns handed out placards – reading take, swing, bunt, etc. – to fans and allowed them to make managerial decisions for a day. Taylor dutifully surveyed the fans' advice and relayed the sign accordingly.

Veeck also brought the legendary, and seemingly ageless, Satchel Paige back to major league baseball to pitch for the Browns. Veeck had previously signed the former Negro leagues great to a contract in Cleveland in 1948 at age 42, amid much criticism. At 45, Paige's re-appearance in a Browns' uniform did nothing to win Veeck friends among baseball's owners. Nonetheless, Paige ended the season with a respectable 3–4 record and a 4.79 earned run average.