Leo Burnett was born in St. Johns, Michigan, on October 21, 1891 to Noble and Rose Clark Burnett. Noble ran a dry goods store and as a young man, Burnett worked with his Father, watching Noble as he designed ads for the Business. After high school, Burnett went on to study journalism at the University of Michigan and received his bachelor's degree in 1914.

Burnett's first job out of college was as a reporter for the Peoria Journal Star in Peoria, Illinois. In 1917, he moved to Detroit and was hired to edit an in-house publication for Cadillac Clearing House, later becoming an advertising Director for the same institution. At Cadillac, Burnett met his advertising mentor, Theodore F. MacManus, whom Burnett called "one of the great advertising men of all time". MacManus ran the agency that handled Cadillac's advertising.

In 1918, Burnett married Naomi Geddes. The couple met at a small restaurant near the Cadillac offices, where Naomi worked as a cashier. They went on to have three children: Peter, Joseph and Phoebe.

After spending a decade at McKee's, and working through the stock market crash of 1929, Burnett left the company. In 1930, he moved to Chicago and was hired by Erwin, Wasey & Company, where he was employed for five years.

A private company formed in 1935 and officially running under the name of 'Leo Burnett Company, Inc.', the agency started with working capital of $50,000, eight employees and three clients. Now a part of Publicis Groupe, Leo Burnett is one of the largest agency networks with 85 offices in 69 countries and 9,000+ employees.

In 1947, Burnett wrote The Good Citizen, a booklet concerning the duties and privileges of being a U.S. citizen. This was done as a public Service for The Advertising Council and The American Heritage Foundation.

Burnett used dramatic realism in his advertising, the Soft sell approach to build brand equity. Burnett believed in finding the "inherent drama" of products and presenting it in advertising through warmth, shared emotions and experiences. His advertising drew from heartland-rooted values using simple, strong and instinctive imagery that talked to people. He was also known for using "cultural archetypes" in his copy, by creating mythical creatures that represented American values. This is evident on such campaigns as Jolly Green Giant, Tony the Tiger, Pillsbury Doughboy and more famously the Marlboro Man. Indeed these campaigns played on the 1950s attitudes towards masculinity that pervaded his campaigns.

In December 1967, nearing the end of his career, Burnett delivered his famous "When To Take My Name Off The Door" speech at the agency's annual holiday gathering.

On June 7, 1971, Burnett went to his agency, pledging to his colleagues to cut back to working only three days per week due to some health problems. That evening, at the age of 79, he died of a heart attack at his family farm in Lake Zurich, Illinois.

During World War I, Burnett joined the Navy for six months. However, his Service was mostly spent at Great Lakes building a breakwater. After his time in the military, Burnett returned to Cadillac for a short while. It was then when a few employees at Cadillac formed the LaFayette Motors Company – triggering Burnett to move to Indianapolis to work for the new establishment. Soon after, he was offered a position at Homer McKee. He then left LaFayette and joined McKee, where Burnett said of the founder, "(He) gave me my first feel of what I have come to regard as the "warm sell" as contrasted to the "hard sell" and "soft sell". This was his first agency job.