Born in Allegheny City, Pennsylvania, which is now part of Pittsburgh, she was the daughter of a well-to-do businessman. Cassatt grew up in an environment that valued education. Her parents believed travel was a way to learn, and before she was 10 years old, she visited many of the capitals of Europe, including London, Paris, and Berlin.

Despite her family's objections to her becoming a professional artist, she began studying painting at the Pennsylvania Academy of the Fine Arts in Philadelphia, Pennsylvania (1861-1865). Tired of patronizing instructors and fellow male students, and the slow pace of her courses, she decided to study the old masters on her own and in 1866 she moved to Paris.

Returning to the United States at the outset of the Franco-Prussian War, she lived with her family, but art supplies and models were difficult to find in the small town. Her father continued to resist her vocation, and paid only for her basic needs but not her art supplies. She returned to Europe in 1871 when the archbishop of Pittsburgh commissioned her to paint copies of paintings in Italy, after which she traveled about Europe.

By 1872, after studying in the major European museums, her style matured, and in Paris, she studied with Camille Pissarro.

The jury accepted her first painting for the Paris Salon in 1872. The Salon critics claimed that her colors were too bright and that her portraits too accurate to be flattering to the subject.

Upon seeing pastels by Edgar Degas in an art dealer's window, though, she knew she was not alone in her rebellion against the Salon. "I used to go and flatten my nose against that window and absorb all I could of his art," she wrote to a friend. "It changed my life. I saw art then as I wanted to see it."

She met Edgar Degas in 1874, and he invited her to exhibit with the impressionists and her work hung in the 1879 impressionist show. An active member of the impressionist circle until 1886, she remained friends with Degas and Berthe Morisot.

Shortly after her triumphs with the impressionists, Cassatt quit painting to care for her mother and sister, who fell ill after moving to Paris in 1877. Her sister died in 1882, but her mother regained her health, and Cassatt resumed painting by the mid-1880s.

Her style evolved, and she moved away from impressionism to a simpler, straightforward approach. By 1886, she no longer identified herself with any art movement and experimented with a variety of techniques. Nearly one third of her paintings depict a mother and child portrayed in intimate relationship and domestic settings.

In 1891, she exhibited a series of highly original colored prints, including Woman Bathing and The Coiffure, inspired by the Japanese masters shown in Paris the year before. (See Japonism.)

The 1890s were Cassatt's busiest and most creative time. She also became a role model for young American artists who sought her advice. Among them was Lucy A. Bacon, whom Cassatt introduced to Camille Pissarro. As the new century arrived, she served as an advisor to several major art collectors and stipulated that they eventually donate their purchases to American art museums. Although instrumental in advising the American collectors, recognition of her art came more slowly in the United States.

After a trip to Egypt in 1910, she was awed by the ancient art. Mary Cassatt's brother, Alexander Cassatt, (president of the Pennsylvania Railroad from 1899 until his death) died in 1906. After her brother's death, she did not paint until 1912.

Diagnosed with diabetes, rheumatism, neuralgia and cataracts in 1911, she did not slow down, but after 1914 she stopped painting because of near blindness. Nonetheless, she took up the cause of women's suffrage, and in 1915, she showed 18 works in an exhibition supporting the movement.

In recognition of her contributions to the arts, France awarded her the Légion d'honneur in 1904.

She died on June 14, 1926 at Château de Beaufresne, near Paris, and was buried in the family vault at Mesnil-Théribus, France.

Before 2005, her paintings sold for as much as $2.8 million.