According to a brief biography in the Idaho Daily Statesman, Knapp was born in Buffalo, New York, where she lived until the age of 27. She displayed a precocious interest in journalism, but was obliged to support herself at the age of 17 and "accepted a commercial position in a large mercantile house." At the age of 24, however, she returned to journalism as associate editor for the Buffalo Christian Advocate. She also attended the medical department of the University of Buffalo for three years.
In 1887, she left Buffalo for San Francisco and she took a job at the San Francisco Call, where she founded the paper's Women's Department. Soon after her arrival, however, she purchased the Alameda County Express, which she ran for a year and a half before merging with the Oakland Daily Tribune and returning to The Call as coast and foreign exchange editor. When her skills as a horsewoman and cattle driver were discovered, however, she was transferred to the livestock department. On her investigative excursions, the article reported, "Miss Knapp uses a small English racing saddle and rides astride, as she finds such a saddle and such a position much less fatiguing than those ordinarily adopted by women."
In April 1891, Knapp met the writer Charlotte Perkins Stetson (later Gilman), who had separated from her husband and recently moved to California. The two women soon became extremely close, and in September, they began living together at 673 Grove Street. "The pleasure in the new relation is that I now have some one to love me, and whom I love" Gilman wrote in The Living of Charlotte Perkins Gilman. "It is possible, but not certain, that Charlotte and delle were lovers," writes Gilman's biographer Ann Lane. The most explicit evidence that they were comes from Stetson's letter to her future husband: "Adeline Knapp has (I suppose she has) letters of mine most fully owning the really passionate love I had for her....I told you that I loved her that way. You ought to know that there is a possibility of such letters being dragged out some day...Fancy San Francisco papers with a Profound Sensation in Literary Articles! Revelations of a Peculiar Past! Mrs Stetson's Love Affair with a Woman. Is this Friendship! and so on." (quoted in Lane, 166)
For reasons that are not entirely clear, Stetson's relationship with "Della" soon became difficult. In her autobiography, Gilman describes her breakup with "Dora" (as she is called in the autobiography) in very harsh terms: "Harder than everything else to me was the utter loss of the friend with whom I had sincerely hoped to live continually. She certainly did love me, at first anyway, and had been most generously kind with money. My return was mainly in service, not only in making a home for her, but in furnishing material for her work. She was a clever writer, and later I learned that she was one of those literary vampires who fasten themselves on one author or another with ardent devotion, and for the time being write like them. The kindest thing I can say of her character is that she had had an abscess at the base of the brain, and perhaps it had affected her moral sense. I do not mean to describe her as 'immoral' in its usual meaning; she was malevolent. She lied so freely as to contradict herself in the course of a conversation, apparently not knowing it. She drank--I saw her drunk at my table. She swore freely, at me as well as others. She lifted her hand to strike me in one of her tempers, but that was a small matter. What did matter was the subtle spreading of slanders about me, which I cannot legally prove to have come from her, but which were of such a nature that only one so close could have asserted such knowledge. Also, I do know of similar mischief-making from her in regard to others. At any rate that solace ended not only in pain but in shame--that I should have been so gullible, so ignorant, as to love her dearly....so the New Year's inscription for 1893 is a doleful one...." (Living, 143-4)
Knapp has not left her own version of events, but there is little in her published writings to support the "malevolent" image sketched by Gilman. Knapp's books, journalism, and activist writing appear to be the work of an active, practical woman with a keen interest in nature and humanity.
In later years, Knapp covered international events such as the Overthrow of the Hawaiian Kingdom and spent time in the Philippines after the Spanish American War, authoring a history book about the Philippines for school use. She was also the author of a novel, The Well in the Desert.
She died in California in 1909 after a long illness. Her New York Times obituary quoted a recent letter summing up her career: "They said I made a hit, (in Hawaii,) but the experience convinced me that newspaper work does not offer a real career for a woman--the sacrifices are too great." For this reason, Miss Knapp wrote, "I went away from cities altogether and lived for two or three years alone in a canyon in the Contra Costa foothills. I built a house there, a small one, all myself, cut down trees, tramped the woods, wrote a book or two, and did a lot of thinking."