In 1823, Lamar's family connections helped him to gain a position as the private secretary to Georgia Governor George M. Troup. In this position, Lamar issued press releases and toured the country giving speeches on behalf of the governor. On one of his trips, he met Tabatha Jordan, whom he married in 1826.
When Troup lost his reelection bid in 1828 Lamar established a newspaper in Columbus, Georgia, the Columbus Enquirer. This venture was much more successful than his previous business attempts. The year after its establishment Lamar's wife died, and he withdrew his name from consideration for election to the Georgia Senate and began to study law. He was admitted to the bar in 1833 and ran an unsuccessful campaign for a seat in the U.S. Congress.
He was also a Master Mason, and a member of Harmony Lodge No. 6 in Galveston, TX.
His actions led to an appointment as the Secretary of War in the ad interim Texas government, and, in 1836, he was elected vice-president of the Republic of Texas under Houston.
His first actions were against the Indian tribes, and in 1839 his troops drove the Cherokee tribes from Texas. The bloody battle resulted in the death of Houston's friend Chief Bowles, leaving Houston very angry at Lamar. A similar campaign was fought against the Comanche, but although many lives were lost, the Comanches could not be forced from the area. Lamar's view towards Native Americans as a whole became increasingly strenuous against tribes, believing that it was necessary to cause their “total extinction.”
Lamar also appointed a commission to select a permanent site for the capital of the Republic. After two months of debate, they recommended the small town of Waterloo along the Colorado River. The town was renamed Austin, and by October 1839 all of the state records and employees had transferred from Houston to Austin. That same year, he also founded the Texas State Library (presently known as the Texas State Library and Archives Commission).
During his administration, Lamar sent three separate agents to Mexico to enact a peace settlement. All of them failed. Nonetheless, Lamar succeeded in gaining official recognition for his country from the United Kingdom, France, and Belgium. Despite several attempts to secure a loan from these and other countries, Lamar was unsuccessful. To fill the treasury, he instead authorized issuance of a large amount of Republic of Texas paper money, known as Redbacks. The paper money was virtually worthless. Spending doubled during Lamar's term as well, and this, combined with the worthless currency, caused severe financial difficulties for the government of the country.
Lamar firmly believed that the Rio Grande River was the western boundary of Texas and wanted to send an expedition to New Mexico to establish trade and possibly convince the residents there to join the Republic. The Texas Congress refused to fund the expedition in both 1839 and 1840. Lamar tired to waiting for permission, and in June 1841 he took $89,000 from the treasury and sent an expedition on his own. The members were arrested when they reached Santa Fe, and, although told they would soon be released, they were marched to Mexico City. Many of the members died during their enforced march.
Lamar is considered the Father of Texas Education. During his administration, he convinced the legislature to set aside 3 leagues of land for each county for equipping schools. An additional 50 leagues of land were set aside for the support of two universities, later Texas A&M University and the University of Texas. Although nothing was implemented during his term, these actions set the foundation for a Texas-wide public school system.
He represented Laredo in the Texas State Legislature for several years after Texas was annexed to the United States. In 1857, President James Buchanan appointed Lamar to be the Minister to Nicaragua. Lamar served in Managua for twenty months before returning to Texas in October 1859 because of health issues. He died of a heart attack on December 19, 1859 and was buried in Richmond.
Lamar University in Beaumont, Lamar State College in Port Arthur, Lamar State College in Orange, Lamar Institute of Technology in Beaumont, Lamar High School in Houston, Lamar High School in Arlington, and Lamar Consolidated High School in Rosenberg are also named for him, as are numerous elementary and junior high/middle schools throughout Texas.
The North Lamar Independent School District, also carries his name, but mostly because it is located in the county named after him.
Lamar also wrote poetry and published his works in a book titled Verse Memorials. He was labelled the "Poet President" and derided by his detractors that he was a better poet than president.
Lamar also served as the first US Ambassador to the Republic of Costa Rica 1857-1859.