At the time of his death, Warner was vice president of the National Prison Congress, president of the American Social Science Association, a trustee of the Wadsworth Athenaeum. He was a charter member and served temporarily as first president of the American Academy of Arts and Letters and sat on the board for city parks in Hartford. Of course, he was also a lively member of several social and literary clubs.
In a eulogy to Warner, a friend noted that his literary success was already publicly known but having him as a friend was far different. "I never heard from his lips an indelicate or coarse story, or an unclean idea," he said. "He abhorred injustice, meanness and dishonor." Further, this friend noted, "We may never see his equal. I cannot expect as equal friendshp and love." Five days before his death, Warner's friend and neighbor Mark Twain had just returned from an extended trip abroad. Twain found Warner's funeral so painful, he chose to move away from Hartford and never lived there again. Instead, Twain moved to nearby Redding, where he died a decade after Warner.
As a writer, Warner mostly focused on nonfiction, but his range of interests was substantial. He wrote about, for example, the role of women in society, the responsibility of writers, and authored several historic and biographical books and essays. In his essay "The Pursuit of Happiness," Warner disputed the titular idea in the Bill of Rights. Instead of chasing after happiness, Warner suggests mankind simply be happy already:
In fine, and in spite of the political injunction, we need to consider that happiness is an inner condition, not to be raced after. And what an advance in our situation it would be if we could get it into our heads here in this land of inalienable rights that the world would turn round just the same if we stood still and waited for the daily coming of our Lord!