He is most remembered for his capture of the notorious border bandit Augustine Chacon in 1902, though he was also a successful businessman who owned the large Diamond A Ranch in New Mexico.

Mossman is said to have joined Theodore Roosevelt's Rough Riders in 1898 and fought in the Spanish–American War; however, author Bill O'Neal makes no mention of this in his Encyclopedia of Western Gunfighters and it seems Mossman was already very busy managing the Hashknife Outfit, serving as sheriff, and pursuing personal business interests.

Mossman accepted and on the following morning the two men met, loaded their weapons with a single bullet, and then took fifteen steps away from each other.

In Walter Canyon, the posse found a cabin and a slaughtered cow, at which time the guide tried to make a run for it with his horse.

One fall night, Mossman was undressing in the second-floor room of a hotel in Springer, New Mexico when a bullet came up through the floor near his chair.

[1] Later that year, Mossman received information that six suspected train robbers were held up twenty miles south of the international border, along the Colorado River in Sonora.

To do this, Mossman came up with an idea that involved posing as an outlaw and recruiting the train robber Burt Alvord, who was a friend of Chacon, to use him as a stool pigeon.

On April 22, 1902, after traveling for several days by wagon and on horseback, Mossman discovered Alvord's hideout, a small hut located some distance away from San Jose de Pima.

The captain approached the hut unarmed and by chance he found Alvord standing alone outside while the rest of the gang were playing cards inside.

When he did finally catch up with Chacon, over three months later, Alvord first had to go with him to the Yaqui River, to sell some stolen horses, before going back to the border.

As the bandits were nearing the rendezvous, Alvord sent Stiles ahead to tell Mossman to meet them just south of the border, at the Socorro Mountain Springs, in Sonora.

There, after exchanging names, Mossman and the others agreed to cross the border back into Arizona on the next day, so they could steal some horses from Greene's Ranch that night.

However, it was decided that it was too dark for stealing horses that night and the party went back to their camp, which was located less than seven miles north of the border.

Of note is that several times Chacon attempted to escape by throwing himself off his horse, presumably at a place where Mossman could not easily follow, such as a steep hillside or something similar.

[3][4] After the capture of Augustine Chacon, Mossman resigned from his position in July 1902 to focus on a peaceful life as a businessman in Bisbee, although rumors circulated that he was uninterested in working for the new governor, L. C. Hughes.

Mossman later returned to the cattle business and purchased the Diamond A Ranch, near Roswell, New Mexico, where he died of old age on September 5, 1956.

Mossman was buried at the Mount Washington Cemetery in Independence, Missouri and the Diamond A Ranch is now listed on the National Register of Historic Places.