He returned to the United States in 1914, married, and in November the couple went to China as missionaries of the American Board of Commissioners for Foreign Missions. Over the following decade, Hummel taught school in Fenzhou, Shaanxi and pursued language studies. He also began to amass a collection of Chinese maps and coins bought in the local markets; his coin collection eventually included 2,000 varieties. In 1924 the Hummels moved to Peking to teach at the newly formed school of Chinese studies at Yenching University. Along with many of the foreign residents, the Hummels left during the unrest of 1927 following the Northern Expedition.