Mastin's grandpa in South Carolina, Reuben, deeded Letita 160 acres of land in Pontotoc County during this time.

On August 15, 1860, the census listed him as an unmarried merchant with $2,000 in real estate and $6,500 in personal property; at that point he also operated a mining business in Piños Altos, Doña Ana County, where gold was discovered in May.

[b] Still practicing law, in December, he discovered a quartz lode along the Continental Divide near Piños Altos,[4] naming it the Pacific.

[4] It also reported him as practicing law in Mesilla County, with his office in Piños Altos, and in the Supreme Court of the Territory of Arizona on February 16, 1861;[5] such was reiterated on March 2.

[6] On March 16, Mastin, representative of Piños Altos, voted in a convention in Mesilla to form Confederate Arizona.

However, on September 27, his brachial artery was severely wounded during the Battle of Pinos Altos while repelling 250–300 Apaches;[8] he died of blood poisoning on October 7.