John Barclay Armstrong, a Texas Ranger known as "McNelly's Bulldog," was born in January 1850 in McMinnville, Tennessee, the son of Dr. John B. Armstrong. After living for a time in Missouri and Arkansas he moved to Texas, in January 1871. He settled in Austin and married Mollie Durst; they had seven children. In the early 1870s Armstrong was a member of the Travis Rifles, and on May 20, 1875, he joined Capt. Leander McNelly's company of Texas Rangers.
Described as well built, with a "full face marked by heavy brows and made distinguished by a finely modeled nose and deep-set languid eyes," Armstrong wore a full beard and was something of a dandy in dress. In 1875 he accompanied McNelly to the Rio Grande and was promoted to the rank of sergeant. He took part in the fighting at Palo Alto Prairie in the so-called Las Cuevas War, and in the fall of 1876 he was involved in the killing or capture of a number of suspected criminals in the area between Eagle Pass and Laredo. In February 1877 he was active in the pursuit of hide thieves on the border, and in December of that year he and a ranger named Deggs killed accused murderer John Mayfield in Wilson County.
When McNelly retired from ranger service, Armstrong was named second lieutenant of the Special Force of Texas Rangers under 1st Lt. Leigh Hall, on January 26, 1877. Several of the company were outspoken in their belief that Armstrong should have been promoted to captain. He was assigned to the Eagle Pass area, where he operated on both sides of the border, assisted in the breakup of several bands of outlaws, and helped arrest John King Fisher in April 1877. While recovering from an accidental self-inflicted gunshot wound suffered at Goliad, Armstrong asked to be allowed to arrest the notorious gunman John Wesley Hardin. The ranger pursued Hardin first to Alabama, then to Florida, then confronted him and four of his gang on a train in Pensacola. In the affray that followed, Armstrong killed one of Hardin's men, rendered Hardin unconscious with a blow from his handgun, and arrested the remaining gang members. After considerable delay in the execution of extradition papers, Armstrong returned Hardin to Texas, where he was tried and sentenced to twenty-five years in prison in September 1877. In July 1878 Armstrong was involved in the killing of outlaw Sam Bass at Round Rock. Thereafter he was stationed for a time at Cuero.
After retiring from ranger service he was appointed a United States marshal. In 1882 he established the 50,000-acre Armstrong Ranch in Willacy County. The old ranger, known in retirement as "Major" Armstrong, died on May 1, 1913.