In the 1690s, the O'odham people living in the area were visited by Eusebio Francisco Kino, who established the nearby Mission San Xavier del Bac.

After Presidio San Agustín del Tucsón was constructed in 1775, sentinels were stationed on the peak to watch for raiding Apaches.

Sentinel Peak is made up of several layers of igneous rock representing various types of volcanic activity, though the mountain itself is not a volcano.

[citation needed] On October 3, 2016, in the Arizona Daily Star newspaper, historian David Leighton explained the origin of the name Sentinel Peak: He wrote that in the very early days of Tucson, an Indian fortification was constructed at the top of a small mountain that would come to be called Sentinel Peak – also sometimes referred to as Picket Post Butte.

"[7] After Arizona's 7–6 victory over Pomona College in 1914, a civil engineering student on the team convinced one of his professors to make a class project of the survey and design for a huge block "A" on Sentinel Peak.

Two weeks later, following much public debate, the Tucson City Council resolved to have the "A" painted red, white, and blue in honor of American troops.

[14] Arizona State University (ASU) has a more recently created "A" Mountain (Tempe Butte) near the school's football stadium.