518 Beacon was designed by Rotch and Tilden, architects, and built by Angus MacDonald, carpenter and builder, in 1893-1894 for Dr. James Bourne Ayer and his wife, Mary Eliza (Farwell) Ayer. Mary Ayer is shown as the owner on the original building permit applications for the stable, dated September 12, 1893, and for the house, dated November 22, 1893, on the final building inspection report for the house dated March 18, 1895, and on the 1895, 1898, 1908, and 1917 Bromley maps.
James Ayer was a physician and maintained his medical office at 518 Beacon. They previously had lived at 53 Mt. Vernon, where he also had maintained his office.
The Ayers’ four children lived with them: Mary Farwell Ayer, Nathaniel Farwell Ayer, James Bourne Ayer, Jr., and Elizabeth Ayer.
James Ayer, Jr., a physician and later a professor of neurology at Harvard Medical School, married in November of 1909 to Hannah Gilbert Palfrey; after their marriage, they lived at 5 Lime. Mary Ayer married in April of 1909 to John Easton Rousmaniere, a lawyer and (after their marriage) a cotton manufacturer in association with his brother-in-law, Nathaniel Ayer. After their marriage, they lived at 352 Walnut in Roxbury.
James Ayer, Sr., died in May of 1909. Mary Ayer and their unmarried children, Nathaniel and Elizabeth, continued to live at 518 Beacon, and James Ayer, Jr., moved his medical office to 518 Beacon (he and his wife continued to live on Lime).
Mary Ayer died in February of 1915. Elizabeth Ayer traveled to Europe in 1917 to work in war relief and reconstruction efforts; she married in August of 1920 (in Bath, England) to Henderson Inches of Boston, a commission merchant. Prior to their marriage, he had lived at 386 Beacon with his mother, Louise (Pomeroy) Inches, the widow of Dr. Charles Edward Inches. After their marriage, Henderson and Elizabeth Inches lived at 39 Pilgrim.
Nathaniel Ayer, a cotton manufacturer, married in May of 1924 to Mrs. Helen Howard (Draper) Taft, the former wife of Walbridge Smith Taft, who had divorced in 1920 and had been living at 297 Commonwealth with her father, George Albert Draper. After their marriage, Nathaniel and Helen Ayer lived at 518 Beacon. They also maintained summer residences in Hopedale and Marblehead. Helen D. Ayer is shown as the owner of 518 Beacon on the 1928 Bromley map. At about the same time as his brother’s marriage, James Ayer moved his medical office from 518 Beacon to 475 Commonwealth (by then, he and his wife lived in Milton).
Nathaniel and Helen Ayer divorced in 1930. Nathaniel Ayer continued to live at 518 Beacon. He is shown as the owner on the 1938 Bromley map.
Nathaniel Ayer died in July of 1948.
By 1950, 518 Beacon was the fraternity house of the MIT chapter of Sigma Phi Epsilon, formed in 1948 and initially called the Pegis Club.
In February of 1953, the Pegis Club was cited by the Building Department for operating a dormitory at 518 Beacon without changing the legal occupancy of the property.
In June of 1955, Sigma Phi Epsilon filed for (and subsequently received) permission to convert 518 Beacon from a single-family dwelling into a fraternity house and dormitory.
518 Beacon remained the Sigma Phi Epsilon fraternity house in 2014.