Kate Wilson was born in 1857 and grew up in New England, most likely in Windsor, Hartford County, Connecticut. She had a younger sister, Fannie Wilson, born in 1868. Her mother died when she was very young, but Kate became very close with her father, Samuel A. Wilson, who was a doctor in Windsor. Some time before 1882, she married Frank V. Mills, a minister of the American Presbyterian church.
The young couple decided to join the missionary efforts of the American Presbyterian Church, North, by embarking on an eight year trip to China. The decade in which they traveled to China was a decade of missionary expansion in that country. As China moved toward modernization, Christian missions increased their number and broadened their focus to include a greater emphasis on education and medicine. The missionaries hoped their Christian influence would shape the way the country modernized. Kate and Frank were part of this effort, and the American Presbyterian Church led the American missionary efforts.
Kate and Frank traveled to China first by traveling west across America, and then crossing the Pacific on the steamer Hiroshima Maru , arriving on November 18, 1882, with several other American families. They arrived in Shanghai and traveled to Hangchow (now called Hangzhou), where they resided for the duration of their stay in China. After attempting to master the language, Frank and Kate held church services in their yard on Sundays. Their close friends Jud and Jennie Judson ran a school for Chinese boys, and Frank assisted in running the school. Kate began a school for girls in 1884, although she found a Chinese prejudice against teaching women kept her enrollment low. Besides doing missionary work in Hangchow, Kate and particularly Frank traveled extensively in the surrounding countryside to preach, but the people in general were more interested in seeing a foreigner than in learning the word of the gospel.
While in China, Kate had two children, but both died at a young age. Her first son, Sidney, was born on October 23, 1884 and died of diphtheria on November 17, 1888. Her second son, Francis Bacon, was born on November 29, 1887 and died of unexplained causes, on July 12, 1888. In part because of the children's deaths, Kate and Frank decided to return to the United States. On April 5, 1889, they left China to return home.