Untermyer, who purchased the estate shortly after Tilden's death in 1886, willed it to the federal, state or Yonkers municipal government to benefit the public.

Eventually, the city agreed to accept part of the estate gardens; this parcel of land was renamed Untermyer Park in his honor.

They had three children, Alvin, who served in the 305th Field Artillery in France during World War I; Irwin Untermyer, a justice of the Appellate Division of the New York Supreme Court, First Judicial Department, and Irene, a philanthropist who married Louis Putnam Myers and, after his death, became the wife of Stanley Richter.

Upon the outbreak of World War I, Untermyer, his wife, and two servants were vacationing in Carlsbad, Austria-Hungary, and returned to the United States aboard the Baltic via London in late August.

[12] He collected art (including in 1892 he acquired Whistler's famous Nocturne in Black and Gold – The Falling Rocket) and dealt with gardening, especially orchid cultivation, and in 1899 bought the former country house of Samuel J. Tildens in Yonkers.

[14] His body was interred (with an accompanying sculpture by Gertrude Vanderbilt Whitney) at a family plot that he established in Woodlawn Cemetery.