At the beginning of the Mexican American War in 1846, he enlisted as a 4th Corporal in Company H of the 2nd Regiment of Pennsylvania Volunteers, known also as the Fayette County Volunteers. As such, he was one of 16 volunteers from Company H to scale the walls of Chapultepec, making it possible for the assault of Mexico City to take place. After Mexico City was taken, Company H was shipped out to New Orleans from Vera Cruz and then went by ship to Pittsburg. After receiving heroes welcomes, the unit was discharged at Uniontown on June 14, 1848. During his period of service in the Mexican-American War, he was promoted to 3rd Corporal, then to Sergeant.
The Mexican War, Ellis History of Fayette Co p 189 The county of Fayette furnished to the United service in the Mexican war one full company of volunteers, raised and commanded by Capt (afterward colonel) William B Roberts, and a large number of men who enlisted in the company of Capt P N Gutherie, both of which commands performed good service on several of the battle fields of that war. The first company mentioned was formed and organized in the fall of 1846, and named the Fayette County Volunteers. It left the county of the 2nd of January, 1847, for Pittsburgh where it was mustered into service on the 4th of the same month and designated Company "H" of the Second Regiment of Pennsylvania Volunteers. The first commander of the company was, as has been stated, Capt Roberts, with William Quail as first lieutenant; but Capt Roberts was soon after promoted to the colonelcy of the regiment and Lieut Quail became captain of "H" Company. The roll of the company as mustered included Fourth Corporal James P Downer.
The Second Regiment, of which the Fayette Volunteers formed a part, proceeded by way of New Orleans to Mexico, and landed at Lobos Island, near Vera Cruz, which was invested by the American forces and fleet. The regiment entered the city after its surrender, and moved thence to the interior by way of Puebla and Perote, being assigned to Quitman's division. During its term of service it took part in the engagements of Tobasco, Cerro Gordo, Contreras, Cherubusco, San Pascual, and at the storming of the Belen Gate (footnote: In the assault on the Belen Gate, Sept 10, 1847, Josiah W Winders of the Fayette company was mortally wounded. He was attended in his last moments by Cyrus L Conner, who promised him that his body should be sent home, and afterwards faithfully kept the promise. The names of others who fell in that assault have not been ascertained.) and it was the first regiment to enter within the walls of the capital. Afterwards it was moved to San Angel. Col Roberts died of disease in the city of Mexico, on the 3rd of October, 1847, and the command of the regiment was assumed by Lieutenant-Colonel John W Geary, afterwards Governor of Pennsylvania. The body of the dead colonel was embalmed, and with that of Lieutenant John Sturgeon of the Fayette County company, was sent back to Uniontown, where the remains were received with every demonstration of sorrow and respect. The funeral procession was escorted by the "Union Volunteers" and the "Fayette Cavalry," Capt James Gilmore. The committee of arrangements (chosen at a public meeting held for that purpose at Uniontown on the 8th of December) was composed of Gen H W Beeson (chairman), John Irons, John M Austin, Dr J Patrick, Armstrong Hadden, E P Oliphant, Dr Smith Fuller, Daniel Kaine, and William Wells. The gentlemen forming the committee of escort were Everhart Bierer (chairman), Col Robert Patterson, Amzi Fuller, Robert S Henderson, M W Irwin, William T Roberts, John Huston, Hugh Graham, John L Dawson, William C Stevens, W R Turner, S D Oliphant, John D Scott, Dr R M Walker, Henry Barkman, William Bailey, D H Phillips, E B Dawson, William Redick, and John Bierer. The remains of Col Roberts were interred in the Methodist graveyard in the borough of Uniontown, and a neat marble monument has been erected over his grave.
With the outbreak of the Civil War, Downer was involved in gathering volunteers from the Junction City area in May and June 1861. They were organized at Lawrence, Kansas, on June 11, and mustered into the army in Kansas City for a 3-month period as the 2nd Regiment of Kansas Volunteers on June 15, 1861 with Downer commissioned as a 2nd Lieutenant. The unit fought at the Battle of Wilson’s Creek, Missouri, on August 10, 1861, which was the first battle of the Civil War west of the Mississippi River. Downer was promoted to 1st Lieutenant on September 1, 1861, and the unit was mustered out on October 31, 1861. He then enlisted as a private in Company C of the 2nd Kansas Cavalry on December 3, 1861. He was wounded in the leg at the Battle of Cane Hill, Arkansas, on November 28, 1862 and was fortunate not to lose his leg to infection. He remained with the unit and was mustered out on January 10, 1865 in Leavenworth, Kansas.