Permanent settlement began in 1847 with the arrival of the first Mormon wagon train, which included three women and three blacks. The advance company planted the first crops on 23 July, and the pioneers at Brigham Young's direction explored nearby areas, built a fort, and surveyed Salt Lake City. In October 1847 seventeen-year-old Mary Jane Dilworth opened the first school in her tent. In the next two years a dozen towns were founded in the county. With self-sufficiency a major goal, the settlers established basic industries to supply everything from pottery to printing paper. They experimented with mixed success in growing many different kinds of plants, raising silkworms, and refining sugar. The county was temporarily abandoned in 1858 during the Utah War. In 1862 U.S. troops established Fort Douglas to protect overland communications and to watch the Mormons.

Tens of thousands of Mormon immigrants funneled through Salt Lake City to outlying settlements, and, additionally, the city was the last major supply point for thousands of California-bound travelers. As the headquarters of the LDS Church and later the territorial and state capital, Salt Lake City and its county have always been the center not only of Utah's population but also of its political and economic power.

Political diversity came to the county in 1870 with the founding of the non-Mormon Liberal party. Until statehood in 1896 the Mormon-Gentile conflict was intense. Industrial development in the late nineteenth and early twentieth centuries brought ethnic diversity with the arrival of many southern and eastern Europeans, blacks, Chinese, Japanese, and Mexicans. The migration of Native Americans to the larger cities and the recent immigration of southeast Asians and others continue the trend.