During the Battle of Monterrey on September 21-24, 1846, Duncan's battery was assigned to Thomas Staniford's brigade in William J. Worth's division.
Taylor attacked the city from the north, while sending Worth's troops to attack from the west.
The batteries of Duncan and William W. Mackall were involved in the capture of the Bishop's Palace on September 22.
Duncan was breveted colonel for "gallant and meritorious conduct" at Monterrey.
In early January 1847, Taylor was ordered to transfer Worth's and Twiggs's divisions and two batteries, including Duncan's, to the army of Winfield Scott.
Duncan fought at the Siege of Veracruz on March 9–29, 1847, at the Battle of Cerro Gordo on April 17–18, and at a skirmish in Amazoque on May 14.

Scott's army was organized into four divisions, with Duncan's battery in Worth's 1st Division.
As the American army approached Mexico City from the east, Scott ordered Worth to reconnoiter a road that would lead the Americans to attack from the south.
Worth gave the task to Duncan who reported the road, "not easily obstructed or defended".
Soon Scott's entire army moved by that route.
At the Battle of Churubusco on August 20, 1847, Newman S. Clarke's brigade, George Cadwalader's brigade, and Duncan's battery captured the tête de pont, a strongpoint defended by a superior force.
They then helped capture the San Mateo Convent.

After a two-week truce, the Battle of Molino del Rey was fought on September 8, 1847.
Underestimating the strength of the Mexican position, Scott assigned the task of capturing the Molino to Worth's reinforced division, a total of 3,500 men.
Worth's soldiers were guided into position by Duncan and James Mason, who had reconnoitered the area.
Worth deployed John Garland's brigade and Benjamin Huger's battery on the right, facing the Molino.
Worth posted Clarke's brigade and Edwin Vose Sumner's 270 dragoons on the left, facing the Casa Mata fortification.
Duncan's battery took position in the center and Cadwalader's brigade was in reserve.
The initial American assault was repulsed with serious losses, but Garland's troops eventually overran the Molino.
Meanwhile, the Mexican defenders of the Casa Mata drove back Clarke's brigade.
A force of Mexican cavalry and infantry appeared to the west, but it was driven off by the fire of Duncan's guns and pursued by Sumner's dragoons.
The Casa Mata repelled a second assault, but its defenders finally fled after a sustained bombardment by Duncan's battery.

Duncan fought during the Battle of Chapultepec on September 13, 1846, and during the subsequent capture of the Belén and San Cosme gates on September 13–14.
During the occupation of the Mexican capital, a quarrel occurred between the American generals.
Gideon Pillow, one of the U.S. division commanders, published an article that appeared in the New Orleans Delta and Picayune newspapers under the pseudonym "Leonidas".
The letter claimed Pillow was responsible for the American victories while Scott was mostly a bystander.
Everyone immediately guessed that Pillow was the author.
On October 23, the Pittsburgh Post published a letter claiming that Worth and Duncan saved Scott from a blunder by convincing the commander-in-chief to use the southern approach to Mexico City.
Actually, Duncan wrote a letter to a friend and this was the basis for the Pittsburgh letter.
Duncan later took credit for the letter.
This was followed by a second letter praising Pillow in glowing terms.
At first, Scott tolerated Pillow's unruly behavior, but he finally had enough and brought charges of insubordination against Pillow, Worth, and Duncan.
During the Battle of Monterrey on September 21-24, 1846, Duncan's battery was assigned to Thomas Staniford's brigade in William J. Worth's division. Taylor attacked the city from the north, while sending Worth's troops to attack from the west. The batteries of Duncan and William W. Mackall were involved in the capture of the Bishop's Palace on September 22. Duncan was breveted colonel for "gallant and meritorious conduct" at Monterrey. In early January 1847, Taylor was ordered to transfer Worth's and Twiggs's divisions and two batteries, including Duncan's, to the army of Winfield Scott. Duncan fought at the Siege of Veracruz on March 9–29, 1847, at the Battle of Cerro Gordo on April 17–18, and at a skirmish in Amazoque on May 14.
Scott's army was organized into four divisions, with Duncan's battery in Worth's 1st Division. As the American army approached Mexico City from the east, Scott ordered Worth to reconnoiter a road that would lead the Americans to attack from the south. Worth gave the task to Duncan who reported the road, "not easily obstructed or defended". Soon Scott's entire army moved by that route. At the Battle of Churubusco on August 20, 1847, Newman S. Clarke's brigade, George Cadwalader's brigade, and Duncan's battery captured the tête de pont, a strongpoint defended by a superior force. They then helped capture the San Mateo Convent.
After a two-week truce, the Battle of Molino del Rey was fought on September 8, 1847. Underestimating the strength of the Mexican position, Scott assigned the task of capturing the Molino to Worth's reinforced division, a total of 3,500 men. Worth's soldiers were guided into position by Duncan and James Mason, who had reconnoitered the area. Worth deployed John Garland's brigade and Benjamin Huger's battery on the right, facing the Molino. Worth posted Clarke's brigade and Edwin Vose Sumner's 270 dragoons on the left, facing the Casa Mata fortification. Duncan's battery took position in the center and Cadwalader's brigade was in reserve. The initial American assault was repulsed with serious losses, but Garland's troops eventually overran the Molino. Meanwhile, the Mexican defenders of the Casa Mata drove back Clarke's brigade. A force of Mexican cavalry and infantry appeared to the west, but it was driven off by the fire of Duncan's guns and pursued by Sumner's dragoons. The Casa Mata repelled a second assault, but its defenders finally fled after a sustained bombardment by Duncan's battery.
Duncan fought during the Battle of Chapultepec on September 13, 1846, and during the subsequent capture of the Belén and San Cosme gates on September 13–14. During the occupation of the Mexican capital, a quarrel occurred between the American generals. Gideon Pillow, one of the U.S. division commanders, published an article that appeared in the New Orleans Delta and Picayune newspapers under the pseudonym "Leonidas". The letter claimed Pillow was responsible for the American victories while Scott was mostly a bystander. Everyone immediately guessed that Pillow was the author. On October 23, the Pittsburgh Post published a letter claiming that Worth and Duncan saved Scott from a blunder by convincing the commander-in-chief to use the southern approach to Mexico City. Actually, Duncan wrote a letter to a friend and this was the basis for the Pittsburgh letter. Duncan later took credit for the letter. This was followed by a second letter praising Pillow in glowing terms. At first, Scott tolerated Pillow's unruly behavior, but he finally had enough and brought charges of insubordination against Pillow, Worth, and Duncan.
James Duncan (United States Army officer)