He worked for his father's real estate agency after leaving school, and took it over completely following his father's death in 1917.
Shearn was elected to the Perth Road Board in 1930, and served as chairman from 1935 to 1936.
He entered parliament at the 1936 state election, winning Maylands from the Labor Party's Robert Clothier.After the 1947 state election produced a hung parliament, Shearn and another independent, William Read, gave their support to the Liberal Party, allowing Ross McLarty to form a government.
The situation persisted after the 1950 election, albeit with the support of another independent, David Grayden.
Shearn died suddenly in January 1951, aged 58.
He had married Emily Ann Watts in 1918, with whom he had two children, Hartley Vivian (Viv) and Marian (Lorraine).