Named for the original Pertido de Ajó district established by Governor Juan Manuel de Rosas in 1839, the fishing village became of interest to tourism during the 1920s, when district Mayor Jorge Gibson graded the area's beaches. The town was officially established on December 21, 1935, and in 1936, its pier was completed; measuring 270 m (886 ft), the Mar de Ajó Pier is the largest of the five in the La Costa District. Mar de Ajó grew to become the most populous of the thirteen La Costa District municipalities, and rivals San Clemente del Tuyú, the northernmost La Costa town, in tourist activity.
Renewed growth in the 1990s led to the establishment of the Universidad Atlántida Argentina in 1994, and a casino in 1996. Other notable installations include the Rubén Luis Di Palma Regional Racetack (among the venues of the TC2000 rally), the Historical Museum and Archives (featuring glyptodons and other cenozoic era fossils), the Parish of Santa Margarita María de Alacoque, and the ship graveyard at Nueva Atlantis – site of the shipwreck of numerous 19th century merchant vessels, notably the Margaretha, a German Empire ship that ran aground in 1880, and whose remains appear during low tide.
The town had a permanent population of 13,769 during the 2001 census [INDEC], and is expected to be home to nearly twice the number for the upcoming 2010 Census.