During the American Civil War, he enlisted in the state's Emergency Volunteer Militia as a private in Landis' Philadelphia Battery at the time of the Robert E. Lee's 1863 invasion of Pennsylvania.

The death of his father, while serving as chaplain of the Fortieth New York Volunteers, obliged him to give up the study of the law.

He published the works of William Dean Howells, Henry James, Mark Twain, and Walt Whitman[4] Gilder took an active interest in all public affairs, especially those which tend towards reform and good government, and was a member of many New York clubs.

He wrote that giving women the right to vote would destroy the "home woman" who was an anchor of family stability in a changing world.

During his service on the commission, he arranged to be called whenever there was a fire in a tenement house, and at all hours of the night he risked his health and his life itself to see the perils besetting the dwellers of the tenements, in order to make wise recommendations as to legislation that would minimize these perils.

[7] Gilder and de Kay were the models for the characters Thomas and Augusta Hudson in Wallace Stegner's Pulitzer-prize winning novel, Angle of Repose.

[8] A celebrated plaster sculpture of the family by Augustus Saint-Gaudens is owned by the Metropolitan Museum of Art.

[9] The Berkshires summer home of Helena de Kay and Richard Watson Gilder is being turned into a museum.

Frank Weitenkampf recounts the following anecdote of Gilder and Alexander Wilson Drake:[11] Gilder died suddenly in New York City on Friday 19 November 1909 of a heart attack in the home of Schuyler van Rensselaer (at 9 West 10th St).

He had been staying at the Rensselaer home as a guest for several days (since the previous Monday, 15 November 1909) with his wife (Helena) when he suddenly died.