His work centers on the experiences of enslaved people, mostly Africans, South Asians, and their descendants, in the cities of colonial Mexico during the 16th and 17th centuries. He is particularly interested in the ways that Africans (especially Angolans) impacted the delicate balance of power in a viceroyalty fearful of a Black rebellion, yet ever more dependent on slave labor. Sierra has recently held a University of Rochester Humanities Fellowship where he completed research on his forthcoming book, Urban Slavery in Colonial Mexico: Puebla de los Ángeles, 1531–1700.