When Konigsburg submitted Mixed‑Up Files to Jean Karl at Atheneum in 1966, she was an unpublished mother of three children living in the suburbs of New York City.
One inspiration for the novel was a page-one story in the New York Times on October 26, 1965.[a] Konigsburg recalled years later that the Metropolitan Museum had purchased for only $225 a plaster and stucco statue from the time of the Italian Renaissance. "They knew they had an enormous bargain."
Another inspiration was complaints by Konigsburg's children in Yellowstone National Park, about a picnic with many amenities of home. She inferred that if they ever ran away "[t]hey would certainly never consider any place less elegant than the Metropolitan Museum of Art".
The author's two younger children Laurie and Ross (who turned eleven and nine in 1967) posed for the illustrations of Claudia and Jamie. Anita Brigham, a neighbor in their Port Chester, New York, apartment house posed as Mrs. Frankweiler.
The character of Mrs. Frankweiler was based on Headmistress Olga Pratt at Bartram's School for Girls in Jacksonville, Florida, where Konigsburg once taught chemistry. "Miss Pratt was not wealthy, but she was a matter-of-fact person. Kind, but firm."
On February 21, 2014, family and friends of E.L. Konigsburg gathered in a private space at the Metropolitan Museum of Art to pay tribute to the author, who died on April 19, 2013 at age 83. One of the speakers was Paul Konigsburg, the author's son. He told a story.
- During the mid-1960s, [Konigsburg] would drop off [her young son] Paul and his siblings, Laurie and Ross, at the museum, while [Konigsburg] attended her own art classes. By the time the children made their routine visits to the knights in armor, the mummy, and the Impressionists (at Laurie's request), Konigsburg's class would be finished and she would return to explore the museum with them.
- On one such occasion, Paul recalled, his mother spotted a single piece of popcorn on the floor next to an ornate piece of royal furniture, which was completely blocked off from public access. He remembers his mother wondering aloud, where did that popcorn come from? And it was that moment, "burned into shrapnel memory", that he believes formed the kernel of the story that would become From the Mixed-Up Files of Mrs. Basil E. Frankweiler. She was "a very special lady", he said, whose passion for art drew her to this "very special place".