On October 18th, 1896, newspaper tycoon William Randolph Hearst published the first issue of the American Humorist, a color supplement to the New York Journal which featured what is considered the first modern comic strip: Hogan's Alley, by Richard F. Outcault:
Set in the tenements of New York’s Lower East Side, the comic chronicles the antics of a group of raucous working-class children. The kids were led by a scrappy young boy whose given name was Mickey Dugan but who quickly became known by the nickname “The Yellow Kid” after the mustard-colored nightshirt that he commonly wore.
— Michelle Ann Abate, "The Yellow Kid and the Yellow Peril"
Hogan's Alley (November 29, 1896). Source: Wikimedia Commons.
The strip was a cultural phenomenon, and "Outcault’s scrappy protagonist became the first merchandized comic strip character, appearing on cracker tins, cigarette packs, ladies’ fans, buttons, and a host of other artifacts” (Abate 2021). Other artists quickly took note: Winsor McCay's Art Nouveau masterpiece Little Nemo in Slumberland premiered in the New York Herald in 1905, followed by Henry Conway Fisher's slapstick humor strip Mutt & Jeff in 1907 (credited as the first comic to use panel borders, rather then simply arranging images sequentially). In 1913, George Herriman debuted Krazy Kat, a strange comic about a love triangle between Ignatz the mouse, Krazy the cat, and friendly Offissa Pupp, set against a background of stark, abstract landscapes.
Krazy Kat (March 20, 1941). Source: Wikimedia Commons.