Brashear was born in Brownsville, Pennsylvania, a town located 35 miles (56 km) south of Pittsburgh along the Monongahela River.

As a young boy, John Brashear was profoundly influenced by his maternal grandfather, Nathanial Smith, who was a clock repairer.

He pursued his passion for astronomy during the night, with the assistance of his wife Phoebe Stewart, a Sunday school teacher whom Brashear met in 1861 and married in 1862.

[1] With limited financial resources to purchase a telescope, Brashear constructed his own workshop using a three-meter-square coal shed behind his house.

He notably developed an improved silvering method, which would later become the standard for coating first surface mirrors, known as the "Brashear Process," until vacuum metalizing began replacing it in 1932.

He founded "John A. Brashear Co." with his son-in-law and partner, James Brown McDowell (now a division of L-3 Communications, and still based in Pittsburgh).

Optical elements and instruments of precision produced by John Brashear were purchased for their quality by almost every important observatory in the world.