UCLA student Colin Higgins wrote Harold and Maude as his master's thesis. While working as producer Edward Lewis's pool boy, Higgins showed the script to Lewis's wife, Mildred. Mildred was so impressed that she got Edward to give it to Stanley Jaffe at Paramount. Higgins sold the script with the understanding that he would direct the film but he was told he wasn't ready, after tests he shot proved unsatisfactory to the studio heads. Ashby would only commit to directing the film after getting Higgins' blessing and then, so Higgins could watch and learn from him on the set, Ashby made Higgins a co-producer. Higgins says he originally thought of the story as a play. It then became a 20-minute thesis while at film school. After the film came out, the script was turned into a novel then a play, which ran for several years in Paris.
Ashby felt that Maude should ideally be European and his list of possible actresses included dames Peggy Ashcroft, Edith Evans, Gladys Cooper and Celia Johnson as well as Lotte Lenya, Luise Rainer, Pola Negri, Minta Durfee, and Agatha Christie. Ruth Gordon indicated that in addition she heard that Edwige Feuillère, Elisabeth Bergner, Mildred Natwick, Mildred Dunnock, and Dorothy Stickney had been considered.
For Harold, in addition to Bud Cort, Ashby considered all promising unknowns, Richard Dreyfuss, Bob Balaban, and John Savage. Also on his list were John Rubinstein, for whom Higgins had written the part, and then-up-and-coming British pop star Elton John, whom Ashby had seen live and hoped would also do the music.
Anne Brebner, the casting director, was almost cast as Harold's mother, when Vivian Pickles was briefly unable to do the role.