Slidell is a city on the northeast shore of Lake Pontchartrain in St. Tammany Parish, Louisiana, United States. The population was 28,7…Slidell is a city on the northeast shore of Lake Pontchartrain in St. Tammany Parish, Louisiana, United States. The population was 28,781 at the 2020 census, making it the sixteenth-most populous city in Louisiana. It is part of the New Orleans−Metairie−Kenner metropolitan statistical area.