The Ruth Fielding series came onto the children's book market in 1913 and lasted for 22 years and 30 volumes. Millie graduated from the University of Iowa, then State University of Iowa, in 1925 with a degree in English. She went to New York after seeing an ad in a trade publication for writers. She met with Edward Stratemeyer, but he did not have any work for her at that time. After going back to Iowa and back to school to get her Masters in Journalism, she was contacted by Edward with the chance to pick up writing books for the Ruth Fielding series. Her first Ruth was Ruth Fielding and her Great Scenario, which was written at Millie's childhood home in Iowa. Writing this book was difficult for her as she could gain no kinship with the character. She worked on the next volume, Ruth Fielding at Cameron Hall, at the university while she was supposed to be at work on her thesis.

Millie wrote the last 8 volumes in the series, volumes 23-30:

• volume #1 Ruth Fielding of the Red Mill, 1913
• volume #2 Ruth Fielding at Briarwood Hall, 1913
• volume #3 Ruth Fielding at Snow Camp, 1913
• volume #4 Ruth Fielding at Lighthouse Point, 1913
• volume #5 Ruth Fielding at Silver Ranch, 1913
• volume #6 Ruth Fielding on Cliff Island, 1915
• volume #7 Ruth Fielding at Sunrise Farm, 1915
• volume #8 Ruth Fielding and the Gypsies, 1915
• volume #9 Ruth Fielding in Moving Pictures, 1916
• volume #10 Ruth Fielding Down in Dixie, 1916
• volume #11 Ruth Fielding at College, 1917
• volume #12 Ruth Fielding in the Saddle, 1917
• volume #13 Ruth Fieldingin the Red Cross, 1918
• volume #14 Ruth Fielding at the War Front, 1918
• volume #15 Ruth Fielding Homeward Bound, 1919
• volume #16 Ruth Fielding Down East, 1920
• volume #17 Ruth Fielding in the Great Northwest, 1921
• volume #18 Ruth Fielding on the St. Lawrence, 1922
• volume #19 Ruth Fielding Treasure Hunting, 1923
• volume #20 Ruth Fielding in the Far North, 1924
• volume #21 Ruth Fielding at Golden Pass, 1925
• volume #22 Ruth Fielding in Alaska, 1926
• volume #23 Ruth Fielding and her Great Scenario, 1927 (Benson)
• volume #24 Ruth Fielding at Cameron Hall, 1928 (Benson)
• volume #25 Ruth Fielding Clearing Her Name, 1929 (Benson)
• volume #26 Ruth Fielding in Talking Pictures 1930 (Benson)
• volume #27 Ruth Fielding and Baby June, 1931 (Benson)
• volume #28 Ruth Fielding and Her Double, 1932 (Benson)
• volume #29 Ruth Fielding and Her Greatest Triumph, 1933 (Benson)
• volume #30 Ruth Fielding and Her Crowning Victory, 1934 (Benson)

The Ruth Fielding series is based on the character of Ruth, an orphan who comes to live with her miserly uncle. Over the years, she prospers in film making, mystery solving, and marries and has a baby. Her adventures as her character grows up and makes her way in life are recounted in all of the volumes. This series has been recommended by many fans and collectors as a good overall series to collect and read. The last few volumes are the hardest to find.