Of Scottish origin, McNab was a "cattle detective" who worked for Hunter, Evans, & Company, which was managed by New Mexico cattleman John Chisum.

With Tunstall's murder and the outbreak of war, McNab took a prominent position in the Regulator chain of command, second only to foreman Richard "Dick" Brewer.

[citation needed] As Morton was believed to have been one of those who killed John Tunstall, his fate looked grim, especially when the Regulators and their prisoners detoured on their way to Lincoln into the Capitan foothills.

[citation needed] McNab was one of the triggermen in the killing of Sheriff Brady and his deputy George W. Hindman in Lincoln on April 1, 1878,[2] and was present three days later when they shot it out with Buckshot Roberts.

Nine miles below Lincoln, they stopped at the Fritz ranch for a break when they were ambushed by Seven Rivers cowboys,[1] and members of the Jesse Evans Gang, all under the direction of newly appointed Sheriff George Peppin.