She went on to become one of the greatest actresses of her day, successful at home as well as on the London stage. She made England her home for several years, becoming friends with the author Geraldine Jewsbury, who is said to have based a character on Cushman in her 1848 novel The Half Sisters.
She made her stage debut in Boston in 1835, in the opera The Marriage of Figaro. The following year she debuted as Lady Macbeth, and her interpretation was more energetic than previous players. After a successful season in New Orleans, she returned to New York City under contract with the Bowery Theatre. She scored a success to rave reviews in Albany, New York, again playing Lady Macbeth.
By 1839, her younger sister Susan Webb Cushman became an actress, and at the age of 14 had married Nelson Merriman. Her husband left her shortly thereafter, pregnant, leaving Charlotte to care for her sister. The two sisters became famous for playing Romeo and Juliet together, with Charlotte playing the part of Romeo Montague.
In 1843, Cushman became involved romantically with Rosalie Sully, a daughter of artist Thomas Sully. By 1844, the romance had ended. She began travelling abroad acting in theater, and Sully died shortly thereafter.
In 1848, Cushman met journalist, writer and part time actress Matilda Hays. The two women became close friends, and after a short amount of time and some correspondence, they became involved in a lesbian affair. For the next ten years the two would be together almost entirely. They became known for dressing alike, and in Europe were publicly known as a couple.
In 1849, Cushman returned to America, but by 1852 she had decided to retire from the stage and took up residence with Hays in Rome, Italy. They began living in an American expatriate community there, made up mostly of many lesbian artists and sculptors of the time. Cushman used her notoriety to promote the works of African American/Native American sculptor Edmonia Lewis, who had become a close friend, and whom Cushman greatly admired.
In 1854, Hays left Cushman for lesbian sculptor Harriet Hosmer, which launched a series of jealous interactions between the three women. Hays eventually returned to live with Cushman, but the tensions between her and Cushman would never be repaired. By late 1857, Cushman was secretly involved with lesbian sculptor Emma Stebbins. One night while Cushman was writing a note, Hays walked in on her. Suspecting that the note was to Stebbins, Hays demanded to see it. Although Cushman maintained that the note was not to Stebbins, she refused to show it to Hays. The altercation that followed was explosive. Hays became enraged, and began chasing Cushman around the house pounding her at every opportunity with her fists. The relationship ended immediately, and Hays moved out. She then sued Cushman stating in her claim that she had sacrificed her own career to support Cushman's career, and therefore was due a certain payment. Cushman paid her an unknown sum, and the two women parted company forever.
Emma Stebbins moved in with Cushman shortly after the break-up. Cushman traveled to America for a short tour a couple of months later. Although Cushman maintained that she was devoted to Stebbins, she became involved with another woman not long after her relationship with Stebbins began. Cushman had met 18 year old actress Emma Crow, and Cushman immediately fell for her. The two women began an affair, and Cushman often called her "my little lover".
Before her departure to Italy, Cushman offered a farewell performance at the Washington Theater in the title role of Hamlet. The poster advertising her appearance describes her as "a lady universally acknowledged as the greatest living tragic actress".
When Cushman returned to Italy, Crow followed. Not long after arriving in Italy, Crow attracted the attention of Cushman's nephew, Ned Cushman. In April 1861, Ned and Emma Crow married.
In 1869, Cushman underwent treatment for breast cancer. Stebbins ignored her own sculpting career and devoted all of her time to caring for Cushman.
In 1915 she was elected to the New York University Hall of Fame.