After Helen Daniels graduated from the University of Minnesota in 1915 she first got involved with Christian outreach through the local Y.W.C.A. In 1919, she served as a missionary in Nanjing with the Presbyterian General Board of Missions. Her husband J. Horton Daniels worked as a doctor in Nanjing after the couple completed school at the Language School of the University of Nanking. After returning to Minnesota from 1927 to 1930, the couple returned to Nanjing. The two happened to be on furlough during the Nanjing Massacre, but they returned in August 1938.