Although most fighting ended in September 1847, there was no formal peace. Santa Anna resigned the presidency, but convening a government in exile took time. Negotiations began in January 1848 by American diplomat Nicholas P. Trist, who defied President Polk by staying in Mexico after his recall.

Trist was willing to accept lands from Texas to California, while returning other occupied areas. After settling monetary disputes and property and citizenship rights for Mexicans in annexed areas, the Mexican Congress sent the treaty to Washington.

Although Polk was outraged by Trist’s defiance, he doubted Mexico would give him any better terms. He asked Arkansan Ambrose Sevier to guide the treaty through the U.S. Senate, eventually leading to Congressional approval. The Mexican Congress passed it as well, although they argued about signing away Mexican territory. Mexican negotiator Bernardo Cuoto said the treaty recovered "the better part of that which is already under control of the conquering army of the United States; it is more exactly an agreement of recovery than an agreement of cession."

On June 12, 1848, the last U.S. Division left Mexico City with Commissioner Ambrose Sevier, who carried the Treaty of Guadalupe Hidalgo in his saddle bags.