An important figure in the mechanization of the shoe industry, his most lucrative idea was to lease his "McKay machines" rather than selling them outright, collecting a small royalty on each pair of footwear made with his equipment.

Upon his death, after providing for his family and mistresses, he left the bulk of his estate to Harvard University as an endowment to support capable professors to train future engineers.

The gift grew to over half a billion dollars and was indirectly responsible for Harvard's inability to merge with MIT in the early 20th century.

[1] At the time, Lynn and other communities around Boston had become the center of the American shoe industry but they still relied on skilled manual labor organized into elaborate putting-out systems.

[1] Ultimately, McKay's company received royalties on billions of pairs of footwear, making $500,000—about 750 kg of gold—a year at the system's height of profitability around 1876.

[6] Ultimately, they spent $120,000 on their improvements[7] and, after George Copeland demonstrated a viable lasting machine at the 1876 Philadelphia World's Fair,[6] another $130,000 attempting to sue him for patent infringement.

[6] They were able to market an automated lasting machine but it was limited to heavy work and useless for the pointed toes then in fashion or for women's shoes in thin leather, which still made up the bulk of sales.

The full transfer of the principal was delayed 36 years until 1949, however, because of life trusts he separately established for his second exwife[a] Minnie Treat, "the prettiest and sweetest young lady the world has produced"[3] and the 36-year-younger daughter of his former housekeeper; for Minnie's two sons allegedly fathered by a Florentine during a period of abstinence in their relationship;[b] for Minnie's mother and sister; and for 13 other women[c] of no apparent relation with whom he negotiated life trusts in consideration of their love and affection,[d] to such an extent that a neighbor complained about him as a "miserible old whore master" filling his house "with loose women under the noses of respectable people".

[4] The inability of Harvard to share the bequest with the Massachusetts Institute of Technology was an important impediment to the repeated attempts of its president Charles William Eliot to merge the two universities.

[1] Invested by the university, his legacy has grown to over $500,000,000 and supports 40 professorships in engineering and applied science,[3] one of the most significant monetary contributions to academic salaries.