When the Robison brothers, Frank and Stanley, took over control of the National League's St. Louis Browns in 1899, they decided to change the name to the St. Louis Perfectos and alter the uniforms, which featured red trim instead of brown.
Legend has it that St. Louis Republic baseball scribe Willie McHale, on Opening Day, overheard a female fan exclaim “what a lovely shade of Cardinal,” in reference to the trim on the uniforms.
McHale ran with it in his column and the Perfectos became the Cardinals in 1900 and forever thence, the name has remained as the club has been closer to perfection than most other franchises.