After his return to Utah in 1875, he married Kirsten Marie Pedersen, who bore him four children before her death in 1887. On 10 December 1886 he married Emma Howell, who had immigrated to Utah from England with her mother and sister in 1885. She bore him three children. On 18 July 1888 he married Emma's sister, Bertha.
In 1876 he began a career that would span forty-two years as a translator, compiler, editor, and historian. His first undertaking was the compilation and translation of the history of Joseph Smith into the Danish language; this became the first foreign-language book published in Utah. He returned to Scandinavia again in 1879 as a missionary but spent most of his time as translator and assistant editor of Skandinaviens Stjerne, and upon the death of mission president Niels Wilhelmsen he became the acting president for six months until a new president arrived.
He returned to Utah in the fall of 1881 and continued his involvement in publishing and history. In 1886 he became a "partial" employee of the LDS Church with a monthly allowance of fifty dollars. One assignment, undertaken in 1888, involved an extensive trip to visit church historic sites in the east, where he conducted interviews and obtained important historical documents. He traveled the following year to various stakes and missions throughout the church to collect records and diaries for the Church Historian's office. This project eventually led to the writing of a manuscript history for each ward and stake. Jenson's valuable work was recognized with a raise of salary to one hundred dollars a month in 1891 and an appointment as a full-time Assistant Church Historian in 1897.
During his sixty-five-year career, Jenson authored 27 books, edited four historical periodicals, compiled 650 manuscript histories and indexes to manuscript histories, wrote more than 5,000 published biographical sketches, more than 2,000 newspaper articles, and gave an estimated 6,000 addresses and speeches on Mormon history throughout the world. He kept a personal diary from the age of thirteen until his death at the age of ninety-one. His reference works, including Church Chronology and the four-volume Biographical Encyclopedia, are still essential tools for Utah and Mormon historians. Perhaps his greatest disappointment was not to be named Church Historian upon the death of Anthon H. Lund in 1921. The position went to the twenty-five-year-old Joseph Fielding Smith, a former assistant to Jenson and son of the recently deceased LDS Church President Joseph F. Smith.
Despite his disappointment, he continued to serve faithfully the cause of Mormonism and its history until his death on 18 November 1941.