Michael Joseph "King" Kelly (December 31, 1857 – November 8, 1894), also commonly known as "$10,000 Kelly", was an American baseball outfielder, catcher, and player-manager in various professional American baseball leagues including the National League, International Association, Players' League, and the American Association. He spent the majority of his 16-season playing career with the Chicago White Stockings and the Boston Beaneaters. Kelly was a player-manager three times in his career – in 1887 for the Beaneaters, in 1890 leading the Boston Reds to the pennant in the only season of the Players' League's existence, and in 1891 for the Cincinnati Kelly's Killers – before his retirement in 1893. He is also often credited with helping to popularize various strategies as a player such as the hit and run, the hook slide, and the catcher's practice of backing up first base. In 1945, Kelly was elected to the Baseball Hall of Fame.

In concluding where to truly give Kelly credit as an innovator, a 2004 book devoted to 19th-century rule bending in baseball—and which came close to exhaustively accounting for all contemporary reporting on various subjects—placed stress on the following: "Kelly's hook slide does sound special, and players probably tried to copy it. Also, he seems to have been the first big leaguer to successfully cut a base (when the usually lone umpire wasn't looking), at least according to the newspaper record." And, "Kelly could have been the first to foul off lots of pitches on purpose. Doing so was a top trick of some Baltimore players of the 1890s. At the turn of the century, that trick was defused when all foul balls began counting as strikes." Kelly holds the record for stolen bases in a season by a primary catcher, with 56 in 1888.

Kelly's autobiography Play Ball: Stories of the Ball Field was published while he was with the Beaneaters in 1888, the first autobiography by a baseball player; it was ghostwritten by Boston baseball writer John J. "Jack" Drohan. Kelly also became a vaudeville performer during his playing career, first performing in Boston where he would recite the now-famous baseball poem "Casey at the Bat", sometimes butchering it. Kelly's baserunning innovations are also the subject of the hit 1889 song entitled "Slide, Kelly, Slide" and a 1927 comedy film of the same name.

Kelly was born in Troy, New York, to Michael Kelly Sr. and his wife Catharine, both Irish immigrants. Upon the outbreak of the American Civil War, his father joined the Union Army, and Mike likely learned to play baseball while living with his mother and younger brother James in Washington, D.C. After the war, his ill father moved the family to Paterson, New Jersey. Kelly's parents apparently died within about five years of each other and he was orphaned by age 18. A best guess, based on tracking Paterson city directories, is that his father died around 1871, and his mother around 1876, when Michael was 18. According to the 1870 census, a Michael Kelly of his age was working in a silk mill in Paterson as of that year.

By 1873, the fifteen-year-old Kelly was good enough to be invited to play baseball on Blondie Purcell's amateur team in Paterson, which played teams throughout the New York metro area, including the Brooklyn Atlantics from the National Association. From 1875 to 1877, he played three seasons as a semi-pro: in Paterson and then other cities.

In 1877, he was with the Paterson Olympics until around June 10, when he joined the Delawares of Port Jervis, New York. In mid-July, a Paterson paper said he had signed with a Springfield, Ohio, team after rejecting a Port Jervis offer of $70 a month (equivalent to $2,067 in 2024). A few weeks later, Port Jervis had not played again when he signed "with the celebrated Buckeye club of Columbus, Ohio."

He made his big league debut in 1878 with Cincinnati. In 1877, Kelly's friend Jim McCormick was signed to play for the Columbus Buckeyes of the International Association, and he recommended that his friend Mike be signed to be his catcher. The year after that, Kelly signed to play for the Cincinnati Reds, then known as the Red Stockings. Although the concept would come later, Mike Kelly was now a major leaguer.

After playing in Cincinnati for two years as an outfielder and backup catcher, Cincinnati and Chicago White Stockings players went on a tour of California. While there, Chicago secured him for 1880, then-Chicago Secretary Albert Spalding doing the signing. Later from San Francisco, Kelly wrote Spalding, who was back in Chicago, "Cincinnati Club has gone back on us. Please send expenses. Am broke." Cincinnati had fallen on hard times by 1879 and released all their players at the end of that season to save having to pay them a last paycheck.