Leason's parents expected him to continue the family tradition of farming, but he showed an early interest in drawing and was apprenticed in 1906 at Sands and McDougall Lithographers in Melbourne where he was soon doing illustrations for advertising.

He studied at the National Gallery of Victoria Art School under Bernard Hall and Frederick McCubbin and associated himself with the Victorian Artists Society after graduation.

In 1914, he began doing book illustration and developed a reputation as an illustrator. He illustrated for a book of poems by Henry Lawson in 1918. He secured a position as chief designer with the commercial publishing firm of Sydney Ure Smith publishers and he moved to Sydney where he worked as a commercial artist and also illustrated for Home magazine. He was political cartoonist at the Sydney Bulletin.

He was also painting and has work now in the War Memorial Museum and the National Library of Australia in Canberra, the Art Gallery of New South Wales and the State Library of Victoria.

In 1923 he returned to Melbourne to work for the Melbourne Herald as chief staff artist, where he developed the Wiregrass cartoon series. His illustrations appeared in Melbourne Punch and Table Talk. During this period, he developed the style of tonalism with Max Meldrum and the beginnings of his opposition to modernism and expressionism.

Faced dwindling employment during the pre-war years, he moved to the United States in 1938 and began work in New York doing commercial illustration with pen and ink illustration of crime and detective stories (New York Daily News, Blue Book Magazine). He did book illustrations (including the The Wizard of Oz by L. Frank Baum) and work for Collier's, Liberty and Saturday Evening Post magazines.

He established a painting school in New York City in 1941 and continued it on Staten Island until 1957. He painted many landscapes and studio studies, now in the collection of the Castlemaine Art Gallery and Historical Museum in Australia.