Fassett studied art in New York City and in Europe, where she stayed for three years. She won a large local following in Chicago for her paintings, but her major success came after she moved to Washington, D.C., in 1875. There the reputation she gained as a hostess enabled her to receive an unprecedented number of political commissions for a woman and to gain a great many prominent sitters for portraits, including three presidents—Ulysses S. Grant (Grant, Ulysses S.), Rutherford B. Hayes (Hayes, Rutherford B.), and James A. Garfield (Garfield, James A.).

In February 1877 Fassett attended and painted a meeting of the Electoral Commission (appointed to settle disputed ballots in four states in the 1876 presidential election) in the Supreme Court chamber. Her finished painting, The Florida Case Before the Electoral Commission, was an astounding piece of work, faithfully depicting some 260 prominent Washington figures engaged in or attending the hearing. The painting was subsequently purchased by Congress to be hung in the Capitol. In later years Fassett took up miniature painting.