William was born in Wilmington, North Carolina, to Reuben and Hannah Loring. He was a fifth great grandson of New England pioneer Deacon Thomas Loring. When he was four, his family moved to Saint Augustine, Florida, where, at the young age of fourteen, he began a military career that spanned fifty years. As a fourteen-year-old, Loring joined the Florida Militia and gained his first combat experience fighting the Seminole Indians in minor skirmishes that would culminate in the Seminole Wars. When he was seventeen, he ran away to fight in the Texas War for Independence, but was soon retrieved by his father and taken home. For the next few years he continued to fight in the Second Seminole War and was promoted to second lieutenant. In 1837, Loring was sent to Alexandria Boarding School in Alexandria, Virginia, completing his secondary education. He attended Georgetown University from 1839 to 1840 and then went on to study law, and was admitted to the Florida bar in 1842. In 1843, he was elected to the Florida House of Representatives where he served from 1843 to 1845. In 1845 he ran unsuccessfully for the Florida Senate.