On March 5, 1879, she married astronomer David Peck Todd, with whom she had one daughter, Millicent. Mabel Loomis Todd had a passionate sexual nature and wrote freely about it. She wrote soon after her marriage: "Sweet communions. Oh joy! Oh! Bliss unutterable" and "A little Heaven just after dinner." In May 1879, the day she got pregnant, she noted: "A very happy few minutes of love in our room."
She later had an affair with Austin Dickinson, the (married) brother of Emily. According to Peter Gay's distinguished book, as reviewed in Time Magazine of January 23, 1984, Todd kissed Austin Dickinson after he had died, kissed "the dear body, every inch of which I know and love so utterly."
She became friends with the Dickinsons, and though she never met Emily Dickinson in person, the two women exchanged letters. After Emily's death in 1886, hundreds of her unpublished poems were discovered, and Todd was chosen as editor by the family. In 1888, her endeavor was joined by Thomas Wentworth Higginson. The first volume of Poems by Emily Dickinson was published in 1890. This version included many alterations by Todd.
Todd was a member of the Audubon Society.