During that time, McClanahan began his newspaper career as a weekend sportswriter for The Dallas Morning News when a friend asked for such help. McClanahan then held a number of sports reporting jobs at the News and its sister paper, The Dallas Journal.
At the age of 35, McClanahan enlisted in the Army Air Corps where he attained the rank of captain. He was discharged in 1946, when he returned to the News where he served as writer, editor and cartoonist on the sports staff. After six months, McClanahan was a full-time cartoonist.
Known as "the father of Southwest Conference cartoon mascots", Bill McClanahan was a highly influential sports cartoonist who popularized the "Grid Gram", a visual box score of a football game. However, his distinguished career as an editorial cartoonist began in 1957 with the retirement of News cartoonist John Knott.
McClanahan and fellow Dallas News cartoonist Jack "Herc" Ficklen happily alternated drawing the editorial cartoon, though they didn't always see eye to eye on political issues. Ficklen was the liberal and McClanahan the conservative, but Ficklen said "It was good for us."
The action from McClanahan's sports cartoons carried over to the editorial page. Cartoonist Bill DeOre said, "He always had something moving or hopping or buzzing in his cartoons. He believed in that and it made him kind of unique."
McClanahan won many awards but noted that the highpoint of his career was winning the Southwest Journalism Award in 1970. He was also honored repeatedly by the National Freedom Foundation.
After retiring in 1973, he published two books and illustrated several others. He also became an avid painter, and his work was exhibited in Dallas. McClanahan died in Dallas in 1981.