The jury accepted her first painting for the Paris Salon in 1872. The Salon critics claimed that her colours were too bright and that her portraits were too accurate to be flattering to the subjects.
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 Degas in 1874, with whom she was to be on close terms throughout his life and he invited her to join the Impressionists at a time when Cassatt was more than ready to cast off the academic conventions of the Salon. Cassatt was to become the only American whose work would appear in the Impressionist exhibitions of 1879, 1880, 1881, and 1886.
She was a great practical support to the Impressionist movement as a whole, both by providing direct financial help and by promoting the works of Impressionists in the USA, largely through her brother Alexander. By persuading him to buy works by Manet, Monet, Morisot, Renoir, Degas and Pissarro, she made him the first important collector of such works in America. She also advised and encouraged her friends the Havemeyers to build up their important collection of works by Impressionists and other contemporary French artists. Her own works, on the occasions when they were shown in various mixed exhibitions in the USA, were very favourably received by the critics and contributed not a little to the acceptance of Impressionism there.
Like Degas, Cassatt became extremely proficient in the use of pastel, eventually painting many of her most important works in this medium. She continued to paint modern women after she joined the Impressionists. By the 1880s, her imagery had become more domestic and interior, but no less modern. Cassatt's women engage in contemporary rituals of womanhood, whether sewing, reading or drinking tea, they exuded a sense of dignity and purpose that challenged conventional notions of decorative femininity. As her style evolved, she began moving away from impressionism to a simpler, more straightforward approach. By 1886, she no longer identified herself with any art movement and experimented with a variety of techniques.
Cassatt is perhaps best-known for her paintings of mothers and children, works which also reflect a surprisingly modern sensibility. Traditional assumptions concerning childhood, child-rearing, and the place of children in society were facing challenges during the last part of the 19th century and women too were reconsidering and redefining their place in modern culture. Cassatt was sensitive to a more progressive attitude toward women and children and displayed it in her art as well as in her private comments. She recognized the moral strength that women and children derived from their essential and elemental bond, a unity Cassatt would never tire of representing.
Mary Cassatt especially liked children, doting on her nieces and nephews and the offspring of friends. Naturalism and sensuality of a pure, elemental, and nonsexual sort are the hallmarks of Cassatt's portrayals of childhood during the 1880s and 1890s. An example is Children on the Shore, which she showed at the last Impressionist exhibition, in 1886. While this seaside subject is unique in her oeuvre, the close-up focus on the pair of toddlers and the firm draftsmanship are typical of the artist's style in the 1880s.