John McShain was born on December 21, 1898 on Appletree Street in Philadelphia. His parents (John and Catherine) had both emigrated from County Derry in Ireland. In the early 1880s, John Sr. had been an apprentice carpenter before coming to Philadelphia in 1882 at the age of twenty-one. John Sr. became involved in the construction business as soon as he arrived in Philadelphia. At first, he worked for his uncle Will who had a small building business. After six years, he established his own firm. By the late 1890s, John McShain Sr. had become moderately well established as a builder of Catholic churches, schools, rectories, and convents. John Sr. and Catherine had four children with John Jr. being the youngest.

John attended St. Ann's School of the Sisters of Mercy Academy in Merion and later at LaSalle High School and St. Joseph's Preparatory School where he was captain of the basketball team. After graduation in 1918, he spent a year at Georgetown University, but he had to withdraw when his father died during his sophomore year.

In 1919, John McShain entered the building business in partnership with his father's former superintendent, William Cochran. McShain spent a term studying drafting at the Drexel Institute of Technology and taught himself bookkeeping and estimating. However, within a year, the McShain-Cochran partnership fell apart and John McShain was in business by himself. In 1922, he submitted a low bid on a parish school, St. Ann's in Phoenixville, Pennsylvania, and shortly thereafter received contracts for St. Michael's Convent in Philadelphia, and a "Working Home for Girls" on North Hancock Street in Philadelphia. Most of John McShain's early projects were church related as he specialized in building Catholic Parochial schools, convents, and rectories.

On May 28, 1927 John McShain married Mary Horstmann, daughter of Ignatius Horstmann, a Philadelphia clothing manufacturer and Union League Republican. In the years after his marriage, John McShain established himself as one of Philadelphia's most important builders. In 1929, he bid successfully on the Board of Education building that was being constructed on the Benjamin Franklin Parkway. In the early 1930s, he began bidding on Washington DC jobs. In 1934, he won contracts for the Library of Congress annex and Bureau of Printing and Engraving building. In the middle 1930s, he opened offices in Washington, Trenton (N.J.) and Baltimore (Maryland). These were years in which Franklin D. Roosevelt's New Deal was resulting in a tremendous expansion of building opportunities in the public sector. Many of John McShain's projects, both in Philadelphia and Washington, were financed with funds made available through the Works Projects Administration. In 1938, John McShain received the contract to build the Jefferson Memorial and the next year he began work on the Franklin D. Roosevelt library at Hyde Park, N.Y. In 1940 he received the contract to build the Pentagon, which at the time was the world's largest office building. Working with project director Leslie Groves, McShain's company completed this job in fifteen months. In 1941, McShain's company built Washington's National Airport which was the nation's largest and most modern at the time.

John McShain was a fixture in Washington for much of the 1940s, 1950s, 1960s, and 1970s. During this period, his firm was one of the ten largest construction companies in the United States. His post World War II projects included the 1950s White House renovation (1950-1951), the Clinical Center for the National Institute of Health (1951), National Shrine for the Immaculate Conception (1955), British Embassy (1958), Veterans Hospital (1961), Army Map Service (1962), Harbour Square Apartments (1963), Columbia Plaza Complex (1964), Housing and Urban Development Building (1965), and John F. Kennedy Center for the Performing Arts (1966).

During this period, McShain continued to maintain a major presence in Philadelphia - as his firm built municipal office buildings, public schools, Catholic colleges, and large apartment complexes. In 1938 he was named chairman of the Philadelphia Zoning Board, a position he held until 1952. In Philadelphia, John McShain was active in Republican Party politics. However, in Washington he maintained both business and social relationships with Democratic Presidents Franklin D. Roosevelt, Harry Truman, and John Kennedy.

In the 1940s and 1950s, as John McShain's construction business prospered he began to make investments in Atlantic City and Philadelphia hotels. He purchased the Ambassador, the Claridge, the Traymore in Atlantic City, and the Barclay in Philadelphia. In 1950, John McShain bought the Indiana Limestone Company in an effort to assure a steady supply of an essential raw material. Many of McShain's buildings were characterized by the use of limestone and reinforced concrete, materials that were becoming widely used in the 1940s and 1950s by contractors like McShain who were making an effort to reduce the costs associated with the use of steel and brick.

After the White House renovation was completed in 1951, John McShain began to have health problems. His doctor recommended that he take up horse racing as a hobby. In 1952 he purchased a stable with four horses in central New Jersey which he called Barclay Stables. McShain's horses were extremely successful during the late 1950s. In 1957, Ballymoss won the Irish Derby, the English St. Leger, the Coronation, the Eclipse Stakes, the King George and Queen Elizabeth, and Arc de Triomphe races. Gladness won the Sunninghill stakes and Ascot Champion Stakes.

In late 1959, John McShain purchased a 25,000-acre estate in Killarney, Ireland. He spent more than three years modernizing the estate, which at first was used primarily as a summer residence. As the years went by John McShain and his wife Mary spent more and more time at Killarney. By the late 1960s, he was spending more than one-half the year in Ireland, as he increasingly relied on his associates Paul Fry and Edward Bernhart to manage the day-to-day aspects of his construction business. McShain, of course, kept in touch with his offices in Philadelphia, Baltimore, Washington, and Trenton with a steady stream of correspondence and telephone calls. However, his inability to directly manage the Columbia Plaza and Kennedy Center projects of the middle 1960s from Killarney, may have contributed to some of the problems that led to protracted litigation and substantial financial loss. During these years John McShain also began to devote an increasing amount of attention to outside interests. He served on the Boards of First Pennsylvania Bank, Beneficial Mutual Savings Bank, Philadelphia Transportation Company, Atlantic City Transportation Company, Rosemont College, La Salle College, St. Joseph's University, Catholic University, and Georgetown University.

John McShain was a deeply religious man and a devout practicing Catholic. When he was in High School he seriously considered studying for the priesthood. His only child, Pauline, entered the Convent of the Holy Child Jesus at New Sharon, Pennsylvania on September 25, 1946 and spent her career as a Nun doing educational work for the Catholic Church. In 1944 John McShain organized John McShain Charities and during the next 45 years he transferred most of his assets to the Charities, which made substantial donations to Catholic colleges and parochial schools, as well as to Philadelphia-area cultural institutions.

John McShain died on September 9, 1989.