The family moved house often, from Allegheny City to Pittsburg, then to Lancaster, Pennsylvania where they bought Hardwick - an 18th century country mansion. Mary began her formal education with a governess, Anita Preble, supplemented by musical and French lessons from her mother. She learned to ride and grew to love country life.

Overwhelmed by grief the family moved back to the United States, again frequently relocating. The US Civil War (1861-1865) barely affected the Cassatt family. Alexander began his career with the Pennsylvania Railroad and Robert Cassatt opened a prosperous brokerage and investment firm - Cassatt and Co.

In 1860 Mary enrolled in the Pennsylvania Academy of Fine Arts of Philadelphia where she studied the rudiments of drawing and anatomy. This didn’t satisfy her. Her passion for painting increased, practicing on her own and with her friend Eliza Hadelman. She told her father of her ambition to go to Europe and “be an artist, a true artist” Despite her fathers’s anger and her mother’s anxiety she was allowed to return to Paris at the end of 1865.

Mary settled in Paris with her American friends: Eliza Haldeman, Howard Robert, Earl Shin, Thomas Eakins and the engraver John Sartain. She studied with Charles Chaplin and took classes with Jean-Léon Gérôme but was very disappointed by their teaching. She copied masterpieces in the Louvre Museum, rubbing shoulders with other artists such as the Manet brothers, Fantin-Latour and Félix Bracquemond, then headed for the countryside outside Paris staying in Courances in 1867 and then Ecouen, where she studied with the ‘genre’ painters Pierre-Edouard Frère and Paul-Constant Soyer. In Villiers-le-Bel she was influenced by Thomas Couture then in 1868 she discovered the Barbizon colony of non-conformist outdoor landscape artists such as Millet, Rousseau and Daubigny.

A year later her painting The Mandoline Player signed Mary Stevenson was accepted and hung alongside her future artist friends: Monet, Degas, Renoir, Pissarro, Bazille and Manet. Her work was rejected again by the Salon in 1869, Mary’s friend Eliza Haldman returned to the United States and a disconsolate Mary set off for a sketching trip in Savoie with a Miss Gordon, a family friend, followed by a winter trip to Rome 1869-1870, with her mother. In 1870 the Salon selected her Une paysanne de Fobello, a painting now lost.

She and Emily arrived in Palma on 15th December 1871 and thanks to Sartain’s letters of recommendation were received warmly by the artist community, notably by the painter and engraver Carlo Raimondi. Mary enjoyed approval and celebrity and her canvas On the Balcony during the Carnaval was accepted by the 1872 Salon.

In October 1872 she set out for Madrid, Spain, to see the Rubens, Greco, Velázquez and Goya masterpieces in the Prado Museum. She visited Saville in the spring of 1873 then returned to Paris for the Salon, successfully entering her painting Torero and Young Girl. In June she went to the Netherlands in search of the works of Van Dyck, Rembrandt and Frans Hals, then to Anvers to meet the engraver Joseph-Gabriel Tourny.

Her Portrait d’Ida was exhibited at the Salon in 1874 and was admired by Degas and Tourny. The following year she submitted two paintings: one was accepted and one refused on the grounds that it was too light colored. In 1876, the year of Mary Cassatt’s last participation in the Salon, this picture was accepted after she darkened the background. Both pictures have disappeared.

On the 10th March, 1879 she took part in the fourth Impressionist exhibition, contributing three pastels, a drawing and seven paintings, including Woman Reading and Woman in a Loge. More a portraitist than a landscape artist, Mary found herself to have an affinity for Impressionist artists with her liking for working outdoors, her sense of color and artistic tastes. She became good friends with Berthe Morisot and the other American painters living in France: Whistler and Sargent.
When Mary was invited to contribute to a new art magazine, Le Jour et la Nuit (never actually published) she took up etching and printmaking.

At the fifth Impressionist exhibition of April 1880, she contributed etchings and eight paintings including Tea, introducing her signature theme of mother and child.

The art dealer Paul Durand-Ruel started to represent her work amidst growing critical acclaim. For the sixth Impressionist Exhibition of 1881 she exhibited pastels and four paintings including Lydia Crocheting in the Garden at Marly and Autumn.

Mary was greatly distressed by these successive bereavements but she continued to paint. In the following years she traveled to Spain with her mother, took part in the last Impressionist exhibition in May 1886, showing six paintings and a pastel, then exhibited two works in the first Impressionist art exhibition in New York organized by Durand-Ruel.

In 1889 she participated in the first Paris exhibition of the society of painter-printmakers. Rich Americans began seeking her advice for their art collections, notably Louisine’s husband, Henry Osborne Havemeyer the “sugar king” of New York. She collaborated with Durand-Ruel who had just opened a gallery in New York.

On the advice of Pissarro, she rented the Château de Bachivillers in the Oise for the summer. She received an unexpected commission for the Woman's Pavilion at the Chicago World’s Fair of 1893. She was asked to paint a monumental allegory to the glory of Modern Woman. Mary Fairchild MacMonnies was to paint a facing mural on Primitive Woman. This was a project of Potter and Bertha Palmer who with the support of Louisine Havemeyer enthusiastically lobbied for the dissemination of art in the United States and female emancipation.

After some hesitation at the scale of the commission, she accepted and set up her workshop in the huge woodshed near the château. She even had a ditch dug which was deep enough to enable her to manipulate and paint the triptych. Her mural disappeared after the exhibition and there remains only one painting referencing this monumental work - her canvas Young Woman Picking Fruit which was inspired by the central panel. Cassatt sent the mural to Chicago in 1893 but didn’t attend the inauguration.

In 1894 she bought the Château de Beaufresne in Mesnil-Théribus. She lived there mainly in the summer, until her death in 1926 (thirty years). No more need for summer rentals, she now owned her own country house where she could entertain family and friends and work as and when she desired. She didn’t much change the structure of Beaufresne but installed modern comforts (running water, central heating and bathrooms). She found the experience of trying to get the local workmen to satisfy her exigent requirements to be very frustrating. She laid out the park, planted fruit trees, hundreds of rose bushes and built a vegetable garden.

Miss Cassatt, as she was known by the inhabitants of Mesnil-Théribus, was seen as distant but very generous. She gave toys to the local children at Christmas and subsidized the salary of the village school mistress Mademoiselle Réty (later Madame Riché). According to her favorite model Reine Lefebvre, she suggested providing electricity to the village generated by the old windmill located on her property but this offer was unfortunately refused. She employed Mathilde Valet, her faithful Alsatian maid and companion, a chauffeur, Armand Delaporte, three gardeners and eight farmworkers.

In 1901 she traveled to Italy with the Havemeyers playing the role of expert art advisor, as she had with her friends Alfred and Ada Pope when they were staying with her in Beaufresne. She became good friends with their daughter Theodate Pope, the renowned architect. Mary successfully liaised between her painter friends, the important art dealers and her rich American acquaintances such as Andrew Carnegie, Henry Clay Frick, Cornelius Vanderbilt, Pierpont Morgan, John Davison Rockefeller, Andrew Mellon, John Howard Whittemore and Potter Palmer, to further her goal of getting European art into American museums.

In 1903 the Bernheim-Jeune Gallery exhibited Mary’s work for the first time, with eight paintings, and a Manet and a Morisot from her personal collection. She revisited the Netherlands to admire Rembrandt's work. She was named chevalier of the Légion d’Honneur in 1904 and accepted the Norman Wait Prize from the Art Institute of Chicago for her painting A Carress, 1896.

In 1908 she exhibited some pastels at Vollard’s Gallery and had another solo exhibition at Durand-Ruels’. Later that year she made her last trip back to the US to support Louisine Havemeyer who had just lost her husband.
Depressed by these deaths Mary took an extensive trip to Turkey and Egypt with her brother Gardner’s family, in 1910 and 1911 then rented the Villa Angelotto in Grasse.

The city of Philadelphia showered her with honorary distinctions and when the Pennsylvania Academy of Fine Arts organized an exhibition entitled “Exhibition of Paintings and Drawings by Representative Modern Masters in 1920 a gallery was dedicated to her work.

An exhibition featuring her etchings was shown first in New York then at the Art Institute of Chicago. A misunderstanding about some of her etchings caused a temporary but traumatic rift with Louisine Havemeyer. At the end of her life, she became somewhat cantankerous, coping with loneliness, ill health, the loss of friends and loved ones and an estrangement from family.