Richards was also interested in religion. He became converted as a teenager to his parents' Congregational faith, but when he was seventeen his application for membership was rejected. He consequently developed a skepticism toward organized religion. However, in the summer of 1836, Brigham Young and his older brother Joseph, Willard's cousins, visited the Holliston area as missionaries for the Church of Jesus Christ of Latter-day Saints. They introduced Willard to the Book of Mormon and taught him concerning Joseph Smith and the claims of restoration of the true church. Shortly after their visit, Willard sold all his medicines, closed down his practice, and moved to Kirtland, Ohio. Other members of his family followed him, and on New Year's Eve he was baptized in icy water by Brigham Young.

Being a single man, Richards immediately was called into missionary service. He first went to New York and New England with Brigham Young to visit family members. Then in June 1837 he traveled to England with other missionaries to open a mission in Great Britain. While in England, he served as assistant to the editor of the Latter-day Saints' Millennial Star.

In England, two great events in Richards's life took place. The first was his marriage in 1838 to Jennetta Richards. Two sons were born to them in England; the first died several months after birth, however. The second great event was Willard's call to be a member of the Quorum of Twelve Apostles. He was ordained an apostle in 1840 in England by Brigham Young.