The original Carysfort Reef light was a lightship, starting in 1825. The first lightship was built in New York City. While being sailed to its station, it went aground near Key Biscayne during a storm, and its crew abandoned the ship. The ship was salvaged by wreckers and taken to Key West, Florida. The owners bought the ship back and it was placed on station at Carysfort Reef. The lightship was often blown off-station by storms, and even went aground on the reef at one point. That first lightship had to be replaced after only five years because of dry rot. The second lightship was named Florida.
The lightship keeper had moved his family to a small house on Key Largo, and kept a garden there. After the Cape Florida Lighthouse was burned by Seminoles in 1836, the keeper moved his family to the lightship. The loss of the Cape Florida Lighthouse left the Carysfort Reef lightship as the only navigational light on the Florida coast between St. Augustine and Key West. In 1836 Seminoles attacked the keeper and four of his helpers as they went ashore on Key Largo to tend their garden. The keeper and one helper were killed, and two of the other three were wounded, but the three managed to escape back to the ship.
Congress appropriated funds for a lighthouse at Carysfort Reef in the 1840s. It was the third screw-pile lighthouse in the United States. The interchangeable parts were manufactured in 1848 in Philadelphia, Pennsylvania, and a construction crew was trained there. The erection of the lighthouse was more difficult than expected. The site was under four-and-a-half feet of water,and the reef was not solid, as expected, but consisted of a hard shell over compacted sand. The plans had to be modified by adding large plates to the piles to spread the weight of the lighthouse over a larger area of the reef. When the supervisor of the construction died, the Army Corps of Topographical Engineers sent Lieutenant George Meade to complete the project. This was Meade's first command of a lighthouse project.
The structure originally had a balcony that encircled the enclosed, circular light keeper's quarters. The balcony and its railing were later removed.