After attending the Episcopal Academy, he studied for two years at the Pennsylvania Museum and School of Industrial Art and then became a newspaper illustrator.

He recalled, "My first newspaper work was drawing pictures of gravestones atop the oldest graves in a local cemetery for The Philadelphia Record.

(The cartoonist Joe Doyle drew both Scary William and The Irresistible Rag after Knerr left these strips.)

[3] Knerr took over The Katzenjammer Kids Sunday strip in November 1914 when Dirks left the Hearst-owned New York Morning Journal after a legal dispute.

[5] Knerr's continuation of The Katzenjammer Kids has been praised as "a particularly brilliant job... true to the spirit of the original, and yet stylistically his own.

On July 8, 1949, he died in Manhattan from heart disease, survived by a sister in Carmel, California, and a brother in Philadelphia.