In 1914, the 17-year old Cowley was called to serve as a missionary. His original assignment was to serve in Hawaii, like his older brothers. However, his assignment was changed, and he went to New Zealand instead. There, he developed an unusual talent with the Māori language and a love of people. He revised the Māori translation of the Book of Mormon, which was then published in 1917. He also translated the Doctrine and Covenants and the Pearl of Great Price into Māori, with the assistance of Wiremu Duncan and Stuart Meha. The translated version of these scriptural texts was published in 1919. His mission was extended to five years to complete these tasks. He returned from his mission to Salt Lake City in 1919; however, he continued to write letters to the Maori people throughout his life, and would later return to New Zealand.