In 1851, while living in Copenhagen, Gudmundson heard LDS Church missionaries Peter O. Hansen and Erastus Snow preaching Mormonism.
He was baptized into the LDS Church on February 15, 1851, by Hansen, and on April 18, 1851, he was given the Aaronic priesthood and ordained a teacher by Snow.
Eventually, he was given the Melchizedek priesthood by one of his missionary companions, after which he baptized nine individuals into the LDS Church.
While emigrating to Utah, Niels Garff, one of Gudmundson's Danish converts, grew ill and died.
In the early 1860s, Gudmundson and his wife became involved with the Church of the Firstborn, a schismatic sect of Latter Day Saints led by Joseph Morris.
They were excommunicated from the LDS Church, and Gudmundson became a member of the Quorum of the Twelve Apostles of the Morrisites.