As a teenage cartoonist, Hubenthal began working with the likes of George Herriman, Will Gould, and the great sports cartoonist, Willard Mullin -- whom he had always considered his "mentor".

During high school in 1934, Hubenthal's track coach introduced him to Mullin, then the sports cartoonist for the Los Angeles Herald Express who would later pen the famous 'Brooklyn Bum' character.

Mullin offered Hubenthal advice and counsel and the pair gradually developed a lifelong friendship. In fact, when speaking of the younger Hubenthal's sports cartoons, Mullin would say that "they look mine on a good day."

He served in the Marine Corps in World War II and worked after his discharge as a commercial illustrator in New York and Los Angeles. In 1949, he became one of the last people personally hired by William Randolph Hearst, when the legendary publisher persuaded him to return to the newspaper business as sports cartoonist for the Hearst publishing chain. "Old W. R. was also convinced I could become a political cartoonist", recalled Hubenthal.

In 1955, Hubenthal became editorial cartoonist for the Los Angeles Examiner, and the Hearst chain distributed his political cartoons nationally for the next 33 years. He continued at the Herald-Examiner until his retirement in 1982, sharing editorial cartooning duties during the last few years with Bill Schorr -- ironically, a former "student" of Hubenthal's who was hired by the Herald-Examiner from the Kansas City Star.

"Hubenthal's cartoons display an amazing command of composition, anatomy and his media," Richard Marschall wrote in The World Encyclopedia of Cartoons. "His work is handsome and easy to look at; Hubenthal can make a hard-hitting point or elicit a chuckle with equal ease."

In a 1982 Herald-Examiner article commemorating his retirement, Hubenthal said "The nice part of what I do is that I get instant responses. I either get panned, or praised, but at least I know what I'm saying is having some effect. When you don't get mail, that's when you should worry." Hubenthal also claimed that his mail ran about 40% in favor of his work, 60% against. "But that's human nature", he said. "People who generally agree with you are not too moved to write. It's the people who become exasperated with my view who usually write."   Long a fan of Hubenthal's work, President Johnson is presented with an original caricature by the cartoonist in the Oval Office

Hubenthal is remembered for his kindness and generosity. Steve Greenberg, a cartoonist with the Seattle Post-Intelligencer, recalled that Hubenthal  "was the first pro to see my fledgling cartoons at the Los Angeles Daily News, and was always willing to offer advice to someone just starting out." To freelance cartoonist Bob Staake, "Hubenthal was like a God to me. He was so kind, so generous with his advice and counsel, and I will always remember with great fondness the times I spent talking cartooning with him in his Encino studio. For a sixteen-year-old kid who wanted to become a professional cartoonist, nothing more firmly sealed that destiny than when I became friends with Karl in 1974."

After his retirement, Hubenthal pursued a successful career in Fine Arts, showing his paintings in galleries in Laguna Beach, California, and in Arizona. His paintings and drawings are in the private collections of eight former U.S. Presidents, as well as various personalities in national politics and sports.