Piasecka Johnson was born in Staniewicze near Grodno, Poland (now in Belarus). Her Father was a farmer. She graduated from Wroclaw University with a M.A. in Art History. She left Poland in 1968, with $100 in her possession.

During this time J. Seward Johnson' engaged in extramarital affairs with Piasecka Johnson. In 1971, they married with none of Johnson's children in attendance. According to trial witnesses brought to court by J. S. Johnson's children during the proceedings concerning his will, Piasecka Johnson "often physically and emotionally abused her husband". Piasecka Johnson said she was a devoted wife for 12 years of marriage.

Johnson was involved in a number of charitable projects, especially in Poland, through the Barbara Piasecka Johnson Foundation which she established in 1974. Johnson founded a school in Gdańsk for autistic children, and donated to the church in Poland, and welfare centres.

She was the primary beneficiary of his will and received the bulk of her husband's fortune of $400 million after he died in 1983 ($983 million as of 2017). Excluded from their father's will, Seward Johnson's six children sued on grounds that their Father wasn't mentally competent at the time he signed the will. The case went to trial but was settled prior to return of a verdict, and the children were granted about 12% of the fortune, leaving Piasecka Johnson in control of the majority of the estate.

Paintings from Piasecka Johnson's art collection were displayed in Monaco's Museum of the Chapel of Visitation from 1995 to 2014. Her collection was removed from the chapel in May 2014 in preparation for its eventual sale at Christie's auction house in July 2014.

In 2007, Johnson was listed on the Forbes 400 World's richest People list with an estimated net worth of $2.7 billion, making her the 149th richest person. She converted the family estate in Princeton, New Jersey, named Jasna Polana, into a country club.