The parish has a total area of 703 square miles (1,820 km²), of which, 648 square miles (1,678 km²) of it is land and 55 square miles (142 km²) of it (7.80%) is water.
The parish is 32 miles long by 30 miles wide. The northern part of the parish consists of rolling terrain covered by pine and hardwood forests about 50 feet above sea level. In the southern end of the parish, the land submerges into rich cypress forests and marshes that border on Lake Maurepas and the Amite River.
As of the census of 2010, there were 128,026 people, and 44,166 households. The population density was 197.5 people per square mile (55/km²). There were 51,607 housing units.