The founders of Ainsdale were probably of Scandinavian origin, driven out of Ireland in the 9th Century. This is borne out by the predominance of Old Norse place names locally, including Ainsdale itself, which was first recorded as Einulvesdel, meaning the valley of a man called Einulfr. The settlers seem to have been left alone by the Anglo-Saxons further inland, probably because the area was not considered to be valuable, as it was largely marshland or windy beaches with no harbours. Ainsdale was mentioned in the Domesday book, valued as 2 carucates of land (an area requiring 16 teams of 8 oxen to plough). Very little changed for hundreds of years, the main occupations were fishing and smuggling from the beach, farming rabbits in the dunes and mixed agriculture inland.
Ainsdale only had any significant development after the railways came in 1848. Houses were built here for the wealthy who commuted into Liverpool and the booming seaside resort of Southport. Shops and services grew up around the railway station to serve the new commuter population. Ainsdale enjoyed a period of popularity as a bathing resort form the late 19th century, but it could never compete with Southport.
In 1912 Ainsdale became part of Southport, along with Birkdale. The village's largest period of growth was during the 1960's and 70's when several large housing estates were built all around the older village centre. In 1841 the population was only 176, by 1981 it had grown to 15,803.