John Leopold was born in Bohemia, now the Czech Republic, in 1890, and took out a homestead in Alberta in 1914. He tried to enlist in the North-West Mounted Police Expeditionary Force to fight the Bolsheviks in Siberia; but he was too small to meet the requirements for the NWMP, and the force was then discriminatory towards most ethnic nationalities. They did recruit him, though, as he spoke five languages, and appointed him to the security intelligence service to spy on labour, socialist, and immigrant organizations. He led a secret life in Regina as Jack Esselwein, a painter, and became active in the Painters' Union, in which he served as secretary from 1922 to 1927. He regularly attended the Regina Trades and Labour Council, and after their formation Esselwein became the secretary of the Communist Party's Regina branch. He also took up work as an insurance agent and a real estate salesman, which allowed him flexibility to travel to and infiltrate organizations in other cities.
In 1927 he was transferred to Toronto, where the stress of his double life led to heavy drinking. The Communist Party exposed him in 1928, and he was transferred to Whitehorse. Leopold resurfaced as a regular member of the RCMP when he stood witness in the trial of the eight jailed leaders of the Communist Party in 1931. He returned to Regina on July 1, 1935, with files that convinced the police to arrest the leaders of the On-to-Ottawa Trek which initiated the Regina Riot. Later he was involved in the early Cold War defection of Igor Gouzenko. Leopold was ultimately promoted to the rank of Inspector in the Research Section of the Intelligence Service. He retired one year later in 1952, and died in Ottawa in 1958.