About the age of 12, newspaper artist J. H. Leonard took Jardine on as an apprentice. He also attended J. S. Watkins's commercial-art classes. He joined the Australian Star (later the Sun), where he mainly worked for the next twenty years. He illustrated The Coloured Conquest by “Rata” (Thomas Richard Roydhouse) published by the NSW Bookstall Company in 1904 and did work for Lone Hand (~1910).

From 1905 to 1917 he was also in partnership with J. B. Jones in an advertising company that employed up to 20 staff.

Around 1908, Jardine traveled internationally to study advertising and illustration, funding the trip through illustration freelancing in the US. He returned to the US in 1923, attracted by high fees for commercial artists. He worked on Hearst’s Cosmopolitan Magazine and Good Housekeeping, The American Legion and Motor Magazine, as well as other magazine covers and commercial work. He worked on booklets and campaign poster.

Due to ill health, Jardine returned to Australia in 1928 and opened a studio in Margaret Street, which secured clients such as Akubra hats, Arnott's biscuits, Stamina Clothing, Toohey and Tooth breweries, and the Department of Defence. He worked on numerous magazines, newspapers, pamphlets, posters and handbills.

A book of his work, The Art of Walter Jardine was published in 1932.

During the 1950s and 1960s he taught by correspondence for the Art Training Institute, Melbourne. In partnership with W. F. Paterson, he formed the company, Walter Jardine Advertising Service (1945-1959, later Jardine, Paterson & Co.). He is known for his distinctive poster art and his paintings of Australian History. He contributed general illustrations to the Bulletin.

From 1962 to 1964, in retirement, he designed a series of Australian postage stamps featuring explorers such as Cook, Stuart, Tasman, Flinders, Bass and Dampier.