Lucy Mix Day was born February 23, 1879, the 9th of the 11 children of Franklin E. and Mary (Grimes) Mix. In 1883 she accompanied her parents and nine siblings on the journey west which led them to Moscow, Idaho, where her father established an orchard containing many acres of prune, pear, cherry and apple trees. She attended primary and secondary schools in Moscow and also attended the University of Idaho.

She met Jerome Day in 1897 when he, with other cadets from the University of Idaho, went horseback riding near her father's orchard. They were married January 21, 1902 and moved to Burke. Late in 1902 Jerome decided to continue his studies in mining chemistry at the university and the young couple returned to Moscow where they maintained their principal residence, although they also had homes in Wallace and Spokane. They were perhaps the most socially active members of the Day family.

In her later years Lucy spent her summers at her home on Lake Coeur d'Alene, and her winters in Palm Springs, California, where she died on February 22, 1969, one day before her 90th birthday. Both of her children, Bernice, born 1904, and Jerome, Jr., born 1911 predeceased her.