Showalter was born in Greene County, Pennsylvania and came to California in 1852. He lived in Horseshoe Bend, Mariposa County where he was a miner. He ran for and won a seat in the California State Assembly 6th District in 1857–1858 and 1861–1862. In the heat of the struggle to elect a new Senator for California he insulted another state assemblyman, Charles W. Piercy from San Bernardino County, who challenged him to a duel.
Despite dueling being officially illegal in California at the time, it proceeded nonetheless on Saturday afternoon, May 25, 1861, near the residence of Charles S. Fairfax, three miles west of San Rafael in Marin County. The weapons chosen were rifles, to be fired at a distance of forty yards. The first fire was ineffective and Showalter demanded another. On the second fire, Showalter shot Piercy in the mouth and killed him. This was the last of the duels between political figures in California.
Now a fugitive as a result of the duel, Showalter made his way south to Los Angeles County, joining with friends and fellow secessionist sympathizers who wanted to go east to join the Confederate Army. This party was caught at Minter Ranch on November 29, 1861, by a 1st Volunteer Cavalry patrol under Second Lt. C. R. Wellman from Camp Wright, in the mountains southwest of the Warner's Ranch, in the San Jose Valley of eastern San Diego County. On December 10, fearing a large rescue party was coming from El Monte, the commander of Camp Wright sent them under guard to Fort Yuma. Eventually they were released after swearing alliegence to the Union. Showalter then went on to Texas where he was made lieutenant colonel of the 4th Arizona Cavalry Regiment.
In March 1864, Lt. Col. Daniel Showalter joined the force under John Salmon Ford and led his regiment in fighting around Brownsville, Texas, to drive the Union troops out of South Texas. After the Union troops evacuated, Showalter occupied Brownsville, June 30. When his commander, General Ford, fell ill, Showalter was found drunk and unable to command the army. On May 12, 1865, General Day attacked Showalter at Palmito Hill, Showalter was drunk again and unable to command his unit and the enemy artillery caused the 4th Arizona to panic without leadership. George Henry Giddings came up with his battalion, rallied the disorderly unit and stabilized the defense some miles east of Palmito Hill. Showalter was immediately relieved of his command of the 4th Arizona and it was given to Major F. E. Kavanaugh.
- Theodore Henry Hittell, History of California p.279
- Records of California men in the war of the rebellion 1861 to 1867 By California Adjutant General's Office
- T. R. Fehrenbach, Lone Star, Da Capo Press, 2000. pg. 385, 387
- Arizona Brigade
- Arizona Brigade
- Mariposa County History and Genealogy Research site: Mariposa Free Press and Mariposa Gazette Vitals 1863 - 1923; March 10, 1866, Mariposa Free Press, DEATH OF COL. SHOWALTER.