[1] On September 13, 1861, McNelly enlisted in the Confederate States Army, joining Company F of the Fifth Regiment of Texas Mounted Volunteers under General Thomas Green.

After dark, McNelly and his 40 troops marched back and forth across a long bridge that led to the city, shouting as if they were speaking to unseen generals and colonels.

[2] On July 1, 1870, Governor Edmund J. Davis organized a Texas State Police force, naming McNelly one of its four captains.

[3] The Democratic Party regained control of Texas in 1873, and in 1874, to combat massive lawlessness, the newly elected governor, Richard Coke, created two branches of the Texas Rangers, a Frontier Battalion under the command of major John B. Jones, and a designated Special Force, commanded by McNelly, financed by cattle ranchers.

McNelly's special group had the specific task of bringing order to the Nueces Strip, a hotbed of cattle thievery and banditry, where Juan Cortina, the Mexican military chief for the Rio Grande frontier, was conducting periodic guerrilla operations against the local ranchers.

McNelly and 40 Rangers arrived in Clinton, Texas, on August 1 and remained for four months to ensure that Taylor and the witnesses against him lived through the trial.

[6] There is a contemporary report that six members of McNelly's unit were engaged in a gunfight with unknown parties six miles from Clinton on the Yorktown Road, which resulted in one missing, one wounded, and two horses killed.

[6] McNelly's methods had been questioned throughout the years, and although he recovered many cattle stolen from the Texan ranches while aggressively dealing with lawlessness on the Mexican border, he had also gained a reputation of taking part in many illegal executions, and confessions forced from prisoners by extreme means.

McNelly also made himself famous for disobeying direct orders from his superiors on several occasions, and breaking through the Mexican frontier for self-appointed law enforcement purposes.

His actions proved to be effective, however, and he was responsible for putting an end to the troubles with Mexican bandits and cattle rustlers along the Rio Grande that were commonplace during the 1850–75 period.

This gang was headquartered at Camargo, Mexico, directly across the border from the US Cavalry outpost of Ringgold Barracks, near Rio Grande City.

Although notable as rustlers, Fisher's band rarely raided US civilian populations, concentrating more on rustling cattle from their Mexican counterparts across the border.

Spying the Rangers, the Mexicans took flight, driving the herd before them at a frenzied pace, until they reached a little island in the middle of a salt marsh.

Two hundred and sixty-five head of stolen stock were rounded up and eventually returned to their rightful owners in the neighborhood of the King Ranch country.

Under cover of brush and scrub oak, they made their way on foot to General Juan Flores Salinas' stronghold at the Rincon de Cucharas outpost of the Las Cuevas ranch, which in English means "The Spoon Corner."

Later that afternoon, Major A. J. Alexander from Ringgold Barracks arrived with a missive from Colonel Potter at Fort Brown, on the Rio Grande at Brownsville, urging McNelly to retreat.

After a needed night's sleep, Captain McNelly moved his men directly opposite Camargo on the Texas side of the Rio Grande.

Thus, in another invasion of Mexico, twelve or thirteen Rangers, not including McNelly – though accounts differ – crossed the river in a rowboat.

[8] In the episode of the NBC TV series Tales of Wells Fargo entitled "Sam Bass" (1957) the character Captain McNelly is played by Ray Teal.

[citation needed] Don Meredith guest stars as the title character in "Shanklin," an episode of the TV series The Quest.