He began his acting career on stage in his 20s, and in film when he was 49, which coincided with the beginning of film's transition from curiosity to commercial art form. Hart's stage debut came in 1888 as a member of a company headed by Daniel E. Bandmann. The following year he joined Lawrence Barrett's company in New York and later spent several seasons with Mlle. Hortense Rhéa's traveling company. He toured and traveled extensively while trying to make a name for himself as an actor, and for a time directed shows at the Asheville Opera House in North Carolina, around the year 1900. He had some success as a Shakespearean actor on Broadway, working with Margaret Mather and other stars; he appeared in the original 1899 stage production of "Ben-Hur." His family had moved to Asheville but, after his youngest sister Lotta died of typhoid fever in 1901, they all left together for Brooklyn until William went back on tour.