MGM was created by the merging of three smaller studios. Metro Pictures was founded in 1915 by Richard Rowland, and made films in both New York and Hollywood. Goldwyn Pictures was founded a year later by Broadway producers Edgar and Archibald Selwyn and film producer Samuel Goldfish (who would legally change his name to Goldwyn, only to later be forced out). Louis B. Mayer Pictures was founded in 1918 by one of Richard Rowland's partners when Mayer left New York to establish his own company in Hollywood.
Marcus Loew, the original theater chain magnate, bought the first two studios, Metro and Goldwyn, in 1924, in order to have film product to supply his many theaters. Being based in New York, however, Loew needed someone to run his new California acquisitions, and brought in Louis B. Mayer, making him a vice president of Loew's. Loew died three years later, and never saw the great success that MGM became. After a legal battle following Loew's death, Louis B. Mayer took control.
All three studios contributed valuable pieces to make MGM the powerhouse. Metro was considered the dominant member of the trio, based on amount of production, but the Goldwyn Studios in Culver City became (and remained, with great expansion) the headquarters and main production facility. The Goldwyn symbol of Leo the Lion and motto of "Ars Gratia Artis" (art for art's sake) would now belong to MGM. And Mayer Pictures, of course, contributed to the legend himself. Louis B. Mayer would usher MGM into the golden age of film.