Her father's school closed, and afterwards she attended Unitarian minister Cyrus Peirce's school for young ladies. Later, she worked for Peirce as his teaching assistant before she opened her own school in 1835. She made the decision to allow nonwhite children to attend her school, a controversial move as the local public school was still segregated at the time. One year later, she was offered a job as the first librarian of the Nantucket Atheneum, where she worked for 20 years.
Using a telescope, she discovered "Miss Mitchell's Comet" (Comet 1847 VI, modern designation is C/1847 T1) on October 1, 1847, at 10:30 pm. Some years previously, King Frederick VI of Denmark had established gold medal prizes to each discoverer of a "telescopic comet" (too faint to be seen with the naked eye). The prize was to be awarded to the "first discoverer" of each such comet (note that comets are often independently discovered by more than one person). Maria Mitchell won one of these prizes, and this gave her worldwide fame, since the only previous women to discover a comet were the astronomers Caroline Herschel and Maria Margarethe Kirch.
She became the first woman elected Fellow of the American Academy of Arts and Sciences in 1848 and of the American Association for the Advancement of Science in 1850. In 1881, reporting to the Association for the Advancement of Women, Mitchell expressed surprise that no women had been elected to the American Academy of Arts and Sciences after her. Mitchell was also one of the first women elected to the American Philosophical Society (1869, at the same meeting Mary Somerville and Elizabeth Cabot Agassiz were elected). She later worked at the U.S. Nautical Almanac Office, calculating tables of positions of Venus, and traveled in Europe with Nathaniel Hawthorne and his family.
She became professor of astronomy at Vassar College in 1865, the first person appointed to the faculty. She was also named as Director of the Vassar College Observatory. After teaching there for some time, she learned that despite her reputation and experience, her salary was less than that of many younger male professors. She insisted on a salary increase, and got it. She taught at the college until her retirement in 1888, one year before her death.
Mitchell began recording sun spots by eye in 1868, but from 1873, her students and she at Vassar College were able to make daily photographic records, allowing more accurate records. These were the first regular photographs of the sun, and they allowed her to explore the hypothesis that sun spots were cavities rather than clouds on the surface of the sun. For the total solar eclipse of July 1878, Mitchell and five assistants travelled with a 4-inch telescope to Denver for observations.
Mitchell never married, but remained close to her immediate family throughout her life. After she retired from Vassar College in 1888, she lived in Lynn, Massachusetts, with her sister Kate and her family. Very few of her personal documents remain from before 1846. The Mitchell family believes she witnessed personal papers of fellow Nantucketers blown through the street by the Great Fire of 1846, and because fear of another fire persisted, she burned her own documents to keep them private.
Mitchell died of brain disease on June 28, 1889, at the age of 70, in Lynn, Massachusetts. She was buried in Lot 411, in Prospect Hill Cemetery, Nantucket. The Maria Mitchell Observatory in Nantucket is named in her honor. The observatory is part of the Maria Mitchell Association in Nantucket, which aims to preserve the sciences on the island. It operates a natural history museum, aquarium, Maria Mitchell's Home Museum, and the Science Library, as well as the observatory. She was also inducted into the US National Women's Hall of Fame, and was made a National Women's History Month Honoree for 1989 by the National Women's History Project. She was the namesake of a World War II Liberty ship, the SS Maria Mitchell. New York's Metro North commuter railroad (with its Hudson Line endpoint in Poughkeepsie near Vassar College) has a train named the Maria Mitchell Comet in her honor. On August 1, 2013, the search engine Google honored Maria Mitchell with a Google doodle showing her in cartoon form on top of a roof gazing through a telescope in search of comets.
After attending Elizabeth Gardener's small school in her earliest childhood years, Maria attended the North Grammar school, where william Mitchell was the first principal. Two years following the founding of that school, when Maria was 11, her Father built his own school on Howard Street. There, she was a student and also a teaching assistant to her Father. At home, Maria's Father taught her astronomy using his personal telescope. At age 12 1/2, she aided her Father in calculating the exact moment of an annular eclipse.