Marsh moved to Canada in 1930, after being hired as a Director of Social Research at McGill University. McGill was taking part in two American-funded research projects at the time, the Canadian Frontiers of Settlement Project and the Social Science Research Project. Marsh was hired through a grant from the Rockefeller Foundation and named project director for the SSRP. While Director, Marsh published several books on employment in Canada, including Health and Unemployment in 1938. The pivotal text to emerge from Marsh's role as project director was Canadians In and Out of Work; A Survey of Economic Classes and Their Relation to the Labour Market in 1940.
Canadians In and Out of Work was one of the first significant works to analyse class in Canada and remained the most comprehensive study of the subject until John Porter's release of The Vertical Mosaic. The work was not well received by the business community. Marsh and the Social Science Research Project proved to be an irritant to the university and funders and funding was not renewed when the grant ran out in 1940.
Leonard Marsh went on to be named research director for the Committee on Post-War Reconstruction under chair Frank Cyril James in 1941. The resulting report, written by Marsh in a single month 1943, Report on Social Security for Canada laid the foundations for the welfare state in Canada. It is now colloquially known as "The Marsh Report", after Marsh himself. Despite being presented to the committee and subsequently the Mackenzie King government, the report was largely ignored. Some have even postulated that it was a source of embarrassment for the King government.
While largely ignored by the government of the time, Marsh's suggestions (which included but were not limited to unemployment insurance, children's allowances, maternity leave, and government funded health care) would become key parts of the Canadian welfare state.
Leonard Marsh joined the League for Social Reconstruction in 1932 and served as president for two terms, 1937–1938 and 1938-1939.
Marsh was hired by the University of British Columbia's School of Social Work in 1947. In 1959, he was named Director of Research. Marsh joined Faculty of Education in 1964 as a professor of Educational Sociology. He retired in 1972 and was named Professor Emeritus in 1972.
- Health and Unemployment: Some Studies of Their Relationships. Published for McGill University by Oxford University Press, 1938.
- Canadians In and Out of Work; A Survey of Economic Classes and Their Relation to the Labour Market. Published for McGill University by the Oxford University Press, 1940.
- Report on social security for Canada. Toronto: University of Toronto Press, 1943.
- Rebuilding A Neighbourhood; Report on a Deomonstration Slum-Clearance and Urban Rehabilitation Project in a Key Central Area in Vancouver. Vancouver, University of British Columbia, 1950.
- Communities in Canada: Selected Sources. Toronto: McClelland, 1970.
With the League for Social Reconstruction
- League for Social Reconstruction, Research Committee. Social planning for Canada. Toronto: T. Nelson, 1935.
- League for Social Reconstruction, Research Committee. Democracy Needs Socialism. Toronto: T. Nelson, 1938.
- Shore, Marlene. (1987) The Science of Social Redemption: McGill, the Chicago School, and the origins of Social Research in Canada. Toronto: University of Toronto Press.
- Maioni, Andrea. 'New Century, New Risks: The Marsh Report and the Post-War Welfare State in Canada.' Policy Options. August, 2004.
- Horn, Michiel. League for Social Reconstruction: Intellectual Origins of the Democratic Left in Canada 1930-1942. Toronto: University of Toronto., 1980.
- Maioni, Andrea. "New Century, New Risks: The Marsh Report and the Post-War Welfare State in Canada." Policy Options. August, 2004.