John McShain was born in Philadelphia, Pennsylvania to Irish immigrants. McShain's father owned a construction company, which McShain was forced to take over in 1919 when his father died. Under his management John McShain Building, Inc. rose to national prominence.
From the 1930s through the 1960s McShain's company worked on more than one hundred buildings in the Washington, DC area, including the Pentagon, the
Jefferson Memorial, the John F. Kennedy Center for the Performing Arts, the Library of Congress annex, Washington National Airport, and the
reconstruction of the White House.
With his wife, Mary J. Horstmann, McShain purchased and renovated Kenmare House in Killarney, County Kerry, Ireland. In 1978, the couple sold the house to Ireland to be added to Killarney National Park, with reserve on the house and surrounding land for use during their lives.