Amelia was a quiet Spanish settlement during the first Spanish Period because St. Augustine, to the south, dominated the military and trading efforts. During the British Period many of the Spanish settlers left and a few British settlers occupied the area the consisted mostly of a few large plantations. British loyalists fled to there during and after the American Revolution as well as Indians from the Carolina area. When Florida reverted back to the Spanish in 1783, the settlers either gave up their land or declared allegiance to Spain. Spain gradually relaxed its requirements and it again because a quiet community and the town of Fernandina stabilized. In 1807 when the U.S. Congress passed Jefferson's Embargo Act preventing trade with Great Britain, the town of Fernandina and the Amelia Island, Nassau River and St. Mary's River trade blossomed. The area also thrived as a slave import area after the U.S. banned the importation of slaves in 1808. Slavery remained ""legal"" in Florida as late as 1820.
In 1811, the Spanish Governor of Florida, Enrique White, officially named the town on Amelia Island ""Fernandina"" in honor of the Catholic monarch Ferdinand VII. The town flourished and was redrawn in 1812. During the War of 1812, British ships occupied the harbor then U.S. ""patriots"" raided it but later left. The Spanish then decided to build Fort San Carlos in Fernandina in 1816. Independence groups and others again raided the town with varying success and failure. Meanwhile the United States was beginning to negotiate with Spain for the transfer of Florida resulting in the Adams-Onis Treaty of 1819 which was finalized in 1821. Meanwhile Amelia suffered the creation of a competing town on the St. John's River which was later called Jacksonville. Thereafter neither Amelia Island and Fernandina nor the town of St. Mary's on the northern river would dominate the area.