Mary Ellen Pleasant was one of the most fascinating yet little-understood figures of the nineteenth century. In many ways, she was a contradiction of her time. Pleasant defied societal norms to become a successful African American businesswoman and abolitionist. As a black woman who could pass for white, Pleasant worked as a conductor on the Underground Railroad. She helped numerous escaped slaves find their way to Northern states and even brought some as far as Canada. She even secretly provided help and support to John Brown concerning his failed raid on Harper's Ferry. However, the exact nature of her role in this is not entirely known.

Pleasant moved to San Francisco in 1852. She continued her work on the Underground Railroad, helping to bring a number of escaped slaves to California. Still passing as a white woman, she worked in several restaurants, where she learned investment strategies from the patrons. She and her business partner invested in several enterprises, including quicksilver. They did very well for themselves. By the mid-1870s, they had amassed a sizable fortune of around 30 million dollars. This may have made Pleasant the richest African American women of her time. She also purchased and developed a significant amount of land in the San Francisco area.

After the Civil War, Pleasant continued to fight for racial equality. She successfully sued a San Francisco railroad company to desegregate the railroad cars. She also contributed to numerous philanthropies throughout her long life.