Millie wrote the Penny Nichols series under the pen name of Joan Clark for the Goldsmith Company. This series is about Penelope "Penny" Nichols, the daughter of a detective. She helps her father solve cases. Penny Nichols is a similar character to Penny Parker, both more bold and pluckier than even Nancy Drew® was. This is a fun series to read, conisisting of four volumes:

• volume #1 Penny Nichols Finds a Clue, 1936
• volume #2 Penny Nichols and the Mystery of the Lost Key, 1936
• volume #3 Penny Nichols and the Black Imp, 1936
• volume #4 Penny Nichols and the Knob Hill Mystery, 1939

These books came in two sizes--a shorter version and a taller version. Boards came in shades from lime green to brown to red. The same art was featured on each volume's dust jacket of blond Penny and her chapeau-clad chum, Susan.