Bruin is located in northeastern Butler County in the valley of Bear Creek, a tributary of the Allegheny River.

According to the United States Census Bureau, the borough of Bruin has a total area of 1.8 square miles (4.7 km2), all land.

[3] Among the early immigrants who settled in the area soon after the close of the Revolutionary War, were a number of north of Ireland families.

In 1796, when the settlement of this section of Butler County began, several of these families migrated here, becoming the pioneers of the township.

Among those said to have arrived in that year, embracing these north of Ireland families, were a few Germans and a number of native-born Pennsylvanians.

[4] The land was first laid out and surveyed by John Martin in 1837 and was known as Martinsburg, then part of Parker Township.

[5] In 1872 came the tidal wave of oil operators, drillers, pumpers and torpedo men, the population rapidly increasing to 500 or more.

In the fall of that year the decrease in oil production and the smallpox epidemic, which carried away five persons, almost depopulated the village and district.

[4] A small band of Indians continued to reside here for a number of years, and at intervals the fathers of the little tribe would revisit the settlements.

The young Native men, many of whom were doubtless born in the area, frequently returned to hunt deer or panther.

(Junior Order United American Mechanics) was a later organization, and claimed a large membership.