Mary Altha Nims, born in Vermont in 1817, was an American artist. While few records about her personal history remain, she is recognized for her participation in Theorem painting, an artistic method that gained popularity in the early 1800s.

Theorem painting—also known as velvet painting—became a favored decorative art form in the United States during the period in which Nims lived. The technique entailed using stencils to produce images, typically applied to velvet or paper, resulting in soft textures and deep, vibrant colors.

Women and girls, especially in New England, often learned this art in boarding schools and art academies. Mastering Theorem painting was viewed as a key component of an accomplished young woman's education, offering a form of creative expression accepted by the period’s societal standards.

Even though only limited details of Mary Altha Nims’s life are available, her artwork as a Theorem artist sheds light on the creative outlets pursued by women in early nineteenth-century America. Her legacy helps to uphold a distinctive tradition within American folk art and reflects the educational customs of her time.