Swinnerton, working for William Randolph Hearst, developed a comic strip called The Little Bears. It was originally based on some spot illustrations that Frank Noble did as a sort of mascot for the San Francisco Examiner in 1893. Although initially drawn in a fairly realistic style, Swinnerton made the character more cute and cartoony as time went on, and he was given a semi-regular spot in the paper to accompany the weather. The character was launched in a strip in 1895. Children were added to the strip in 1896 and it was renamed Little Bears and Tykes.
In 1898, Swinnerton moved to New York and, evidently at the request of Hearst himself, continued a variation of the strip using tigers instead of bears. Eventually, one of the tigers became the star of the strip and was dubbed "Mr. Jack." The character proved popular enough that he was give his own strip in 1903 titled, appropriately enough, Mr. Jack.
Mr. Jack was, in many respects, a typical-of-the-time caricature of a husband. He was always out drinking, carousing, and flirting with women, often at the protest of his wife. Given the frequent adult nature of the strip, it was moved to the sports section (then considered a more mature section of newspaper than children would not be interested in) within its first year. The strip lasted until early 1906 (though the last year, it was somewhat sporadic). Swinnerton brought the strip back in late 1907 and it ran until 1919. It was brought back a third and final time in 1926, lasting until 1935.
The real significance of the strip, however, is that Mr. Jack is generally considerted the first truely anthropomorphic character in comics. There were previous animal characters that spoke or wore clothing, but Mr. Jack was the first to really be represented in a human-like manner, walking upright with human appendages. Despite being a common sight in comics and cartoons today, perhaps first popularized through George Herriman's Krazy Kat, this simply was not a convention of any sort before Swinnerton. It was Mr. Jack who broke the notion that comic characters were simply exaggerations of people and animals, and they could blur the line between species. Mickey Mouse, Bugs Bunny, and so many other characters that you've loved owed a huge debt to Swinnerton and Mr. Jack.