In 1699, Pierre Le Moyne d'Iberville, a French explorer, was the first European to visit the area of present-day St. Tammany Parish. While exploring lakes Pontchartrain and Maurepas, Iberville wrote in his journal, "The place where I am is one of the prettiest I have seen, fine level ground bare of canes. The land north of the lakes is a country of pine trees mixed with hard woods. The soil is sandy and many tracks of buffalo and deer can be seen."

The first European recorded in the Appalousa territory was the French trader Michel de Birotte. He came in 1690 and negotiated with the Appalousa nation. Nine years later in 1699, France named Louisiana as a colony and defined the land occupied by the Appalousa as the Opelousas Territory. The area south of the Opelousas Territory between the Atchafalaya River, the Gulf of Mexico and Bayou Nezpique, occupied by the Attakapas Indians (Eastern Atakapa), was named Attakapas Territory.

After the founding and development of New Orleans, French settlers began to enter the region. The primary industry was the production of pitch, tar, turpentine and resin from the forests.

After the French were defeated in the French and Indian War, St. Tammany (like the surrounding regions of the Florida Parishes) became part of English West Florida. Then, after Britain was defeated in the American Revolutionary War, West Florida was governed by the British and the Spanish. During the West Florida period, St. Tammany, like the rest of West Florida, attracted British loyalists who wanted to escape persecution in the 13 colonies. The West Florida period ended with the West Florida Revolt, which preceded West Florida's annexation by the United States.

In 1810, President James Madison claimed West Florida as part of Louisiana and sent William C. C. Claiborne to claim the territory. Claiborne established the boundaries of the Florida Parishes. He created St. Tammany Parish and named it after the Delaware Indian Chief Tamanend (c.1628-1698), who made peace with William Penn and was generally renowned for his goodness.[4] Among the nine Louisiana parishes (counties) named for "saints" (see "List of parishes of Louisiana"), St. Tammany is the only one whose eponym is not a saint as recognized by the Roman Catholic Church, the ecclesiastical parishes of which formed the basis for civil parishes prior to statehood. In fact, Tamanend is not known to have been a Christian, and was certainly not a Roman Catholic. However, he became popularly revered as an "American patron saint"[citation needed] in the post-Revolutionary period (long after his death).

In the early 1830s, there were only two towns in St. Tammany: Covington, a retreat with summer homes and hotels; and Madisonville, a shipbuilding and sawmill town. The area south of Covington to Lake Pontchartrain's northern shore and extending eastwards to the Pearl River border with the state of Mississippi was known as the Covington Lowlands. This region included the present-day towns of Mandeville, Abita Springs, Lacombe, Slidell, and Pearl River. Mandeville was founded in 1834 and was developed as a health resort for wealthy New Orleanians, because it was believed that ozone was both salutary and naturally emitted by the numerous trees in the area (both beliefs later proven false), giving rise to an early name for the region — the "Ozone Belt". Regular ferry service commenced across Lake Pontchartrain, and shortly thereafter another resort community was founded, Abita Springs. A railroad was constructed in the 1880s connecting Covington and Abita Springs to Mandeville and to New Orleans, allowing for further growth, particularly in Abita Springs, where underground spring waters permitted supposedly healthful baths.

With the completion of high-speed road connections to St. Tammany from New Orleans and its older suburbs (Lake Pontchartrain Causeway, the I-10 Twin Span), the parish began to develop as a bedroom community. Suburban sprawl first took root in and around Slidell, Louisiana, in the eastern part of the parish. Though the Causeway was completed in 1956 and linked suburban Metairie with western St. Tammany, growth in and around western St. Tammany towns like Mandeville, Covington and Madisonville only gathered momentum in the late 1960s. While St. Tammany was sparsely populated and almost wholly rural in the 1950s, its population exceeded 200,000 in the wake of Hurricane Katrina's landfall in 2005.

A major event in the parish's transition from a bedroom community of commuters to a more diverse and independent economic unit occurred in 2008 with the relocation of Chevron's regional corporate headquarters from downtown New Orleans to an office park outside of Covington.

Hurricane Katrina made its final landfall in eastern St. Tammany Parish. The western eye wall passed directly over St. Tammany Parish as a Category 3 hurricane at about 9:45 AM CST, August 29, 2005. The communities of Slidell, Avery Estates, Lakeshore Estates, Oak Harbor, Eden Isles and Northshore Beach were inundated by the storm surge that extended over 6 miles inland. The storm surge impacted all 57 miles of St. Tammany Parish's coastline, including Lacombe, Mandeville and Madisonville. The storm surge in the area of the Rigolets Pass was estimated at 16 feet, not including wave action, declining to 7 feet at Madisonville. The surge had a second peak in eastern St. Tammany as the westerly winds from the southern eye wall pushed the surge to the east, backing up at the bottleneck of the Rigolets Pass.

The Twin Spans of I-10 between Slidell and New Orleans East were virtually destroyed, and much of I-10 in New Orleans East was under water. The Lake Pontchartrain Causeway and the Highway 11 bridge, connecting the north and south shores of Lake Pontchartrain, were open only to emergency traffic.

Initial search and rescue operations were conducted south of Highway 190 from Lacombe east to the state line. Fire District No. 1 and the St. Tammany Parish Sheriff’s office evacuated over 3,000 people from flooded homes and rescued about 300 people in imminent danger. Radio communications among first responders functioned throughout the rescue period, but the 9-1-1 system was not operational for ten days. Utility services were not available anywhere in the parish. Generator power was available for hospitals and a special needs shelter. Hospitals were running at capacity on generator power.

The hurricane-force winds toppled trees and telephone poles parish-wide, blocking all transportation routes. Land debris cleanup continued into 2007, with over 6.6 million cubic yards collected. Debris cleaning in waterways continued at least through 2009. Hurricane Katrina damaged 48,792 housing units in St. Tammany Parish from flood waters, high winds, or both.

The parish has a total area of 1,124 square miles, of which 850 square miles is land and 279 square miles (24.79%) is water. The population recorded in the 1900 Federal Census was 52,906 . The 2010 census recorded 83,384 residents in the Parish.

Neigboring parishes and counties are Washington Parish (north), Pearl River County, Mississippi (northeast), Hancock County, Mississippi (east), Orleans Parish (south), Jefferson Parish (southwest), and Tangipahoa Parish (west). Communities in the parish include Covington, Mandeville, Slidell, Abita Springs, Madisonville, Pearl River, Folsom, Sun, Eden Isle, and Lacombe.

Birth Records - The Louisiana State office maintains records for 100 years after the date of birth. Birth records are considered confidential for the first 100 years. For current information on who may obtain a birth record as well as how to submit a request visit the Office of Public Health, Vital Records Registry website or write to them at PO Box 60630, New Orleans, LA 70160.

Birth records older than 100 years are available through the Louisiana State Archives, 3851 Essen Lane, Baton Rouge, LA 70809. (225) 922-1000.

Death Records - The Louisiana State office maintains records for 100 years after the date of death. Death records are considered confidential for the first 100 years. For current information on who may obtain a death record as well as how to submit a request visit the Office of Public Health, Vital Records Registry website or write to them at PO Box 60630, New Orleans, LA 70160.

Death records older than 100 years are available through the Louisiana State Archives, 3851 Essen Lane, Baton Rouge, LA 70809. (225) 922-1000.

Marriage Records - For current information on how to submit a request for a certified copy of an Orleans Parish marriage record less than 50 years old, see the Louisiana Office of Public Health Director, Vital Records and Statistics website or write to PO Box 60630, New Orleans, LA 70160.

Certified copies for the parish are issued by Clerk of Court. For the address of the parish Clerk of Court visit the St. Tammany Parish Important Addresses page.

Marriage records over 50 years are stored by the Louisiana State Archives, 3851 Essen Lane, Baton Rouge, LA 70809. (225) 922-1000.

Divorce Records - To obtain current information on how to submit a request for a certified copy of divorce records contact the Clerk of Court. For the address of the parish Clerk of Court visit the St. Tammany Parish Important Addresses page.