Key Allegro is a coastal development located on a triangular peninsula between the communities of Rockport and Fulton in southwestern Aransas County. Cut off from the mainland by Little Bay, the area was used by Karankawa Indians before Anglo settlement. By the late 1860s, with the development of Rockport and Fulton, a hide and tallow factory owned by a man named Cushman operated on the land. In 1874 Bavarian Franz Josef Frandolig obtained a patent from the state of Texas for the property. With his family, he ran a farm and salt works, and the peninsula became known as Frandolig Point. Apparently the Frandolig family was no longer on the site when a severe hurricane swept it clean in 1919. For several decades only fishermen and birdwatchers occasionally visited the area that had become a haven for wildlife.
In 1953 the Aransas County Navigation District purchased from the state 604 acres of "submerged" land in Little Bay. This property, however, included a portion of Frandolig Point, and the Dabney and Ona Petty family, who possessed chain of title back to Franz Josef Frandolig, sued the navigation district over 165 acres of dry land. The Pettys won their lawsuit and in turn sold more than 200 acres of Frandolig Point to the Aransas County Navigation District in the spring of 1955. Initially the district planned to lease the island peninsula to developers for the purpose of constructing resort housing and recreation facilities, but in 1958 district commissioners made plans to put the land up for sale. Carl Krueger, Jr., of San Antonio purchased Frandolig Point in 1961 and renamed it Key Allegro. He envisioned a resort community with 1,000 homes, a country club, and a marina.
Construction had begun by 1962, and work included the dredging of short canals to provide waterfront lots. The dredged material in turn was used as fill to raise the lots, and concrete bulkheads were installed to protect against erosion. A motel and restaurant opened in 1963. In the mid-1960s Krueger even held a short-lived porpoise show as entertainment and publicity for Key Allegro. The popular development grew steadily and what started as a resort area with weekend homes grew into an affluent community. Extensive construction continued throughout the 1970s, and by the mid-1980s Key Allegro reported an estimated population of 600. By the mid-1990s there was no more room for development on the peninsula, and wealthy buyers tore down some older beach houses to construct larger estates. In 2000 the unincorporated community maintained a population of 600.