The Dana Girls Mystery Stories, a Stratemeyer Syndicate creation, was published under the pen name of Carolyn Keene by Grosset & Dunlap. Leslie McFarlane, who wrote many of the original Hardy Boys® books, wrote the first four Dana Girls books. Millie began writing volume #5, The Secret at the Hermitage, published in 1936. She wrote 12 of the Dana Girls books, the last being volume #16, Mystery at the Crossroads, published in 1954. This last Dana Girls book was the last Syndicate volume she authored.

The Dana Girls are sisters Jean and Louise Dana, who attend Starhurst Academy. All the while solving mysteries, they must contend with foe, Lettie Briggs, who is always trying to foil their plans. This series was originally issued with art-deco style dust jackets, later becoming wrap style and then beige spine picture covers. The series stopped being published in 1944 until it began again in 1952. After the last picture cover in 1968, the series was resurrected in the 1970's in white spine picture cover format and a couple of new books were written. Proposed plans for a paperback series in the 1980's did not materialize.

The Mysteries:

• volume#1 By the Light of the Study Lamp, 1934
• volume#2 The Secret at Lone Tree Cottage, 1934
• volume#3 In the Shadow of the Tower, 1934
• volume#4 A Three-Cornered Mystery, 1935
• volume#5 The Secret at the Hermitage, 1936 (Benson)
• volume#6 The Circle of Footprints, 1937 (Benson)
• volume#7 The Mystery of the Locked Room, 1938 (Benson)
• volume#8 The Clue in the Cobweb, 1939 (Benson)
• volume#9 The Secret at the Gatehouse, 1940 (Benson)
• volume#10 The Mysterious Fireplace, 1941 (Benson)
• volume#11 The Clue of the Rusty Key, 1942 (Benson)
• volume#12 The Portrait in the Sand, 1943 (Benson)
• volume#13 The Secret in the Old Well, 1944 (Benson)
• volume#14 The Clue in the Ivy, 1952 (Benson)
• volume#15 The Secret of the Jade Ring, 1953 (Benson)
• volume#16 Mystery at the Crossroads, 1954 (Benson)
• volume#17 The Ghost in the Gallery, 1955
• volume#18 The Clue of the Black Flower, 1956
• volume#19 The Winking Ruby Mystery, 1957
• volume#20 The Secret of the Swiss Chalet, 1958
• volume#21 The Haunted Lagoon, 1959
• volume#22 The Mystery of the Bamboo Bird, 1960
• volume#23 The Sierra Gold Mystery, 1961
• volume#24 The Secret of Lost Lake, 1963
• volume#25 The Mystery of the Stone Tiger, 1963
• volume#26 The Riddle of the Frozen Fountain, 1964
• volume#27 The Secret of the Silver Dolphin, 1965
• volume#28 Mystery of the Wax Queen 1966
• volume#29 The Secret of the Minstrel's Guitar 1967
• volume#30 The Phantom Surfer, 1968