[10] St. Tammany was originally inhabited by numerous Indian peoples, including the Colapissas, Bayou Goulas, Chickasaw, Biloxi, Choctaw and Pensacola nations (although Frederick S. Ellis, in his book St. Tammany Parish: L'autre Côté du Lac, claims that the regionally prominent Choctaw tribe did not arrive in the area until after it had begun to be settled by Europeans).

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.

During this period, the area comprising today's St. Tammany attracted British loyalists who wanted to escape persecution in the Thirteen Colonies.

After Great Britain was defeated in the American Revolutionary War, West Florida was governed by the Spanish.

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.

This region included the present-day towns of Mandeville, Abita Springs, Lacombe, Slidell, and Pearl River.

Mandeville was founded in 1834 by Bernard de Marigny de Mandeville 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.

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 Bridge), the parish began to develop as a bedroom community.

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.

[13] After losing his bid for a sixth term, Strain was the subject of a federal corruption investigation into his privatizing a parish work release program in exchange for kickbacks.

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 storm surge impacted all 57 miles (92 km) of St. Tammany Parish's coastline, including Lacombe, Mandeville and Madisonville.

Initial search and rescue operations were conducted south of U.S. Highway 190 from Lacombe east to the state line.

[20] Radio communications among first responders functioned throughout the rescue period, but the 9-1-1 system was not operational for ten days.

The hurricane-force winds toppled trees and telephone poles parish-wide, blocking all transportation routes.

Hurricane Katrina damaged 48,792 housing units in St. Tammany Parish from flood waters, high winds, or both.

Among the Christian denominations prevalent throughout the parish, Roman Catholicism has remained the largest with 145,007 members, followed by Baptists and non/inter-denominational Protestants per the Association of Religion Data Archives in 2020.