Assumption Parish is an irregular-shaped parish in the south-central portion of Louisiana. It is about 30 miles south from Baton Rouge and 60 miles west of New Orleans. The southern tip of the parish is about 25 miles from the Gulf of Mexico. It is bordered on the north by Iberville and Ascension Parishes, on the east by St. James and Lafourche Parishes, on the south by Terrebonne and St. Mary Parishes, and on the west by St. Martin and Iberia Parishes. The parish has an extreme length of 25 miles and an extreme width of 18 miles. Elevations range from 23 feet above sea level along the high natural levee of Bayou Lafourche near Donaldsonville to near sea level in the swamps south of Lake Verret. The total area of the parish is 236,962 acres, of which 21,654 acres is water. The population of Assumption Parish in 1970 was 19,654. Under French and Spanish rule Assumption formed a part of the "Lafourche Settlement." The first permanent settlements in this region were made by the French and Spanish about the middle of the 18th century along the Lafourche, between the present towns of Donaldsonville and Napoleonville. From 1755 to 1765, the population was increased by the immigration of the exiled Acadians who entered the area clearing the land and building comfortable homes. Many of their descendants are still numerous in the parish today. In 1785, Assumption had a population of 646. By an act of the legislature of Orleans territory in 1807, Assumption was created as the 8th parish of the territory. Napoleonville, situated on Bayou Lafourche, at about the center of the parish, is the parish seat. 

The soil of the entire parish is alluvial and divided into three classes; sandy loam, mixed soil, in which sand and humus are about equal, and black land, in which there is little or no sand. Many thousands of acres are flooded, however, because of their low elevation and lack of adequate outlets. Most of the flooded soils are in woodland. The most valuable land of the parish lies along Bayou Lafourche, extending back from 80 to 100 acres; no better land than this is to be found in the state.


Sugarcane is the main crop. In proportion to its area, Assumption Parish produces more sugar than any parish of Louisiana. With the success of sugar granulation procedures developed in 1749, a few sugarcane plantations were established as early as 1803, but it was not until 1861 that sugarcane became the principal crop grown in the parish. Most of the soils in the parish not subject to flooding have been used for the production of sugarcane. The loamy soils of the parish are well suited to the production of truck and vegetable crops, but at present commercial crops are not produced extensively. However, many home gardens can be found.


Bayou Lafourche is an important Mississippi River distributary that flows southward through the parish from Donaldsonville. The bayou is 107 miles long from Donaldsonville to its mouth emptying into the Gulf of Mexico. The Intracostal Waterway is linked with many navigable bayous on the west side of the parish. The road along the western bank of Bayou Lafourche (Louisiana Highway 1 - LA 1) has been described as the longest street in the world. A large number of houses are built close to each other almost the entire 107-mile length of the bayou. On the opposite bank is State Highway 308. Many bridges cross the bayou to connect Highway 1 and Highway 308. About four miles of U.S. Highway 90, the only Federal highway in the parish, passes through the extreme southern part of the parish. Several oil companies have lines into or through the parish. These lines supply natural gas and oil to many complexes in Louisiana and to other states. Assumption Parish has several landing strips for small aircraft but no commercial air stations.