Answer by taking a quote from the following article:

John Herbert Gleason was born in 1916 at 364 Chauncey Street in the Bedford-Stuyvesant section of Brooklyn. Named Herbert Walton Gleason, Jr., at birth, he was baptized John Herbert Gleason, and  grew up at 328 Chauncey (an address he later used for Ralph and Alice Kramden on The Honeymooners). His parents were Herbert Walton "Herb" Gleason, an Irish-American insurance auditor, and Mae "Maisie" (nee Kelly), originally of Farranree, Cork, Ireland. Gleason was one of two children; his brother Clement J. died of meningitis at age 14.

Gleason worked his way up to a job at New York's Club 18, where insulting its patrons was the order of the day. Gleason greeted noted skater Sonja Henie by handing her an ice cube and saying, "Okay, now do something." It was here that Jack L. Warner first saw Gleason, signing him to a film contract for $250 a week.  By age 24, Gleason was appearing in movies: first for Warner Brothers (as Jackie C. Gleason) in such films as Navy Blues (1941) with Ann Sheridan and Martha Raye and All Through the Night (1941) with Humphrey Bogart, for Columbia Pictures for the B military comedy Tramp, Tramp, Tramp (1942) and finally for Twentieth Century-Fox, where Gleason played Glenn Miller Orchestra bassist Ben Beck in Orchestra Wives (1942). He also had a small part as a soda shop clerk in Larceny, Inc. (1942), with Edward G. Robinson, and a modest part as an actor's agent in the 1942 Betty Grable-Harry James musical Springtime in the Rockies.  During World War II, Gleason was initially exempt from military service, since he was a father of two. However, in 1943 the US started drafting men with children. Gleason reported to his induction where the doctors discovered that his broken left arm had healed crooked, the area between his thumb and forefinger was nerveless and numb, a pilonidal cyst existed at the end of his coccyx, and that he was 100 pounds overweight. Gleason was therefore classified 4-F and rejected for military service.  Gleason did not make a strong impression on Hollywood at first; at the time he developed a nightclub act that included comedy and music. At the end of 1942, Gleason and Lew Parker led a large cast of entertainers in the road show production of Olsen and Johnson's New 1943 Hellzapoppin. He also became known for hosting all-night parties in his hotel suite; the hotel soundproofed his suite out of consideration for its other guests. "Anyone who knew Jackie Gleason in the 1940s", wrote CBS historian Robert Metz, "would tell you The Fat Man would never make it. His pals at Lindy's watched him spend money as fast as he soaked up the booze."  Gleason's first significant recognition as an entertainer came on Broadway when he appeared in the hit musical Follow the Girls (1944). While working in films in California, Gleason also worked at former boxer Maxie Rosenbloom's nightclub (Slapsy Maxie's, on Wilshire Boulevard).

who discovered him
Jack L. Warner first saw Gleason, signing him to a film contract for $250 a week.