Dunn was born in Meadville, Pennsylvania on October 6, 1872. Little is known of his youth, but in 1897, he reached the major leagues as a pitcher for the Brooklyn Bridegrooms. He bounced around the majors for seven years, having one good season with the Bridegrooms in 1899 with a 23-13 record. After 1904, he pitched and managed in the minors for a few seasons before moving into the business side of baseball.
In 1907, Dunn took over as manager of the Baltimore Orioles, a minor league club with no connection to the current major-league team by that name. He bought the team a year later and developed a minor-league powerhouse by scouting and developing his own players.
Dunn first achieved notoriety in 1914, when his Orioles were running away with the league pennant but losing money at the box office because of a rival Federal League team in town purporting to be a major-league club. To make his payroll, Dunn had to move the team to Richmond, Virginia and sell off his star player, Babe Ruth, and 11 other players to the majors.
The team moved back to Baltimore in 1916 and Dunn again put together a juggernaut, ultimately signing 10 more players who went on to have solid major-league careers. The best of these was pitcher Lefty Grove, a future Hall of Famer who went 109-36 as an Oriole between 1920 and 1924. By that time, Dunn's team was in the midst of winning seven straight International League championships, many by huge margins.
Dunn's team was regarded as the equal of many major league teams, and he kept them so by refusing to trade or sell players to the majors. It wasn't until the 1925 off-season, when the other, struggling teams in the league made an agreement with the majors on a set price for transferring players, that Dunn finally relented and began selling his stars for money. His team won one more league title in 1925 and then dropped back into the pack.
Dunn was responsible for Ruth's famous nickname, calling him "my $10,000 Babe" for the price he drew, and in addition to Grove, discovered other quality major-leaguers such as Jack Bentley, Ernie Shore, George Earnshaw, Dick Porter and Tommy Thomas.