When the Mexican–American War began Worth was serving under Zachary Taylor in Texas and negotiated the surrender of the Mexican city of Matamoros.
He next commanded the 2nd Regular Division, Army of Occupation at the Battle of Monterrey in September 1846.
In 1847, Worth was transferred to his old friend Winfield Scott's army and placed in command of the 1st Division.

He took part in the Siege of Veracruz and engaged in the battles of Cerro Gordo, Contreras and Churubusco.
 In Mexico City Scott ordered Worth to seize the Mexican works at the Molino del Rey.
 Worth and Scott's friendship came to a head when Scott refused to allow Worth to modify the attack and the battle caused the 1st Division severe casualties, much to Worth's dismay.
 Worth later renamed his son Winfield Scott to William.
 He next led his division against the San Cosme Gate at Mexico City.
 When U.S. forces entered Mexico City, Worth personally climbed to the roof of the National Palace and took down the Mexican flag replacing it with the Stars and Stripes.

For his service at the Battle of Chapultepec, the United States Congress awarded him with a sword of honor.

In 1847 he was admitted as an honorary member of the New York Society of the Cincinnati.
When the Mexican–American War began Worth was serving under Zachary Taylor in Texas and negotiated the surrender of the Mexican city of Matamoros. He next commanded the 2nd Regular Division, Army of Occupation at the Battle of Monterrey in September 1846. In 1847, Worth was transferred to his old friend Winfield Scott's army and placed in command of the 1st Division.
He took part in the Siege of Veracruz and engaged in the battles of Cerro Gordo, Contreras and Churubusco.  In Mexico City Scott ordered Worth to seize the Mexican works at the Molino del Rey.  Worth and Scott's friendship came to a head when Scott refused to allow Worth to modify the attack and the battle caused the 1st Division severe casualties, much to Worth's dismay.  Worth later renamed his son Winfield Scott to William.  He next led his division against the San Cosme Gate at Mexico City.  When U.S. forces entered Mexico City, Worth personally climbed to the roof of the National Palace and took down the Mexican flag replacing it with the Stars and Stripes.
For his service at the Battle of Chapultepec, the United States Congress awarded him with a sword of honor.
In 1847 he was admitted as an honorary member of the New York Society of the Cincinnati.
William J. Worth