Cornelius McGillicuddy (December 22, 1862 – February 8, 1956), better known as Connie Mack, was an American professional baseball catcher, manager, and team owner. Mack holds records for the most wins (3,731), losses (3,948), ties (76), and games managed (7,755) in Major League Baseball (MLB) history. His victory total is 847 more than the second-highest: Tony La Russa's 2,884 wins. Mack's lead in career losses is even greater, with 1,449 more than La Russa's 2,499. Mack also has 17 more ties than the next-closest manager, Clark Griffith, who has 59.

Mack managed the Philadelphia Athletics for its first 50 seasons of play, starting in 1901; was at least part-owner from 1901 to 1954; and retired after the 1950 season at age 87. He was the first American League manager to lead a team to 100 wins, doing so in 1910, 1911, 1929, 1930, and 1931; his five 100-win seasons are second-most in MLB history, with only two other managers surpassing him. He was the first manager to win the World Series three times, and he is the only manager to win consecutive Series on two occasions (1910–11, 1929–30); his five Series titles remain the third-most by any manager, and his nine American League pennants rank second in league history. However, constant financial struggles forced repeated rebuilding of the roster, and Mack's teams also finished in last place 17 times, including ten seasons in which the Athletics lost 100 games.

Mack was elected to the National Baseball Hall of Fame in 1937.

Mack was born Cornelius McGillicuddy on December 22, 1862, in Brookfield, Massachusetts, in what is now East Brookfield. His parents, Michael McGillicuddy and Mary McKillop, were immigrants from Ireland: Michael from Killarney in County Kerry, and Mary from the Catholic section of Belfast. A wheelwright by trade, Michael served with the 51st Massachusetts Volunteer Infantry Regiment during the American Civil War. He suffered from several ailments as the result of his military service; after the war, he was able to work only infrequently and drew a disability pension.

As with many Irish immigrants whose names began with "Mc", the McGillicuddys were often referred to as "Mack", except in official and legal documents. Michael’s father was named Cornelius McGillicuddy, and by tradition, the family named at least one son in each generation Cornelius. "Connie" is a common nickname for Cornelius, so Cornelius McGillicuddy was called "Connie Mack" from an early age. He did not have a middle name, but many accounts erroneously give him the middle name "Alexander"; this error probably arose because his son Cornelius McGillicuddy Jr. took Alexander as his confirmation name. Connie Mack never legally changed his name; on the occasion of his second marriage at age 48, he signed the wedding register as "Cornelius McGillicuddy". His nickname on the baseball field was "Slats", for his height of 6 feet 2 inches and thin build.

Mack was educated in East Brookfield, and began working summers in local cotton mills at age 9 to help support his family. He quit school after completing the eighth grade at age 14, intending to work full-time to contribute to the family's support, as several of his siblings had done. He clerked at a store, worked on local farms, and worked on the production lines of the shoe factories in nearby towns.

Mack was also a good athlete and frequently played baseball and some of its predecessor games with local players in East Brookfield. In 1879 his skills landed him a place on East Brookfield's town team, which played other town teams in the area. Though younger than his teammates by several years, Mack was the team's catcher and de facto captain.

Beginning in 1886, Mack played 10 seasons in the National League and one in the Players' League, for a total of 11 seasons in the major leagues, mainly as a catcher.