Theodor came to the United States in 1849 and became partner at the New York branch of the English iron firm of Naylor, Benson and Company.
He married Dorthea in 1864 during a visit to Bremen and brought her back with him to the United States, and they lived in a brownstone house in Brooklyn Heights, New York.
[1] Mary was educated by private tutors for the early part of her life, and later took courses at the New York School of Philanthropy.
She lived alone for the rest of her life until dying of a pulmonary embolism on August 15, 1963, at her summer home in Bar Harbor, Maine.
From a financially secure family, she constantly contributed time, funds, and organizing talents to a variety of feminist causes, most notably women workers and the suffrage movement.
In 1915, Mayor John Mitchell appointed her to the New York City Board of Education, but she resigned all posts in 1915 in order to participate completely in the final drive to achieve the vote for women.
In 1917, Dreier became chairwoman of the New York State Committee on Women in Industry of the Advisory Commission of the Council of National Defense.
On the national level she often supported Progressive Party nominees, including Robert M. La Follette and Henry A. Wallace, and she later became an enthusiastic backer of Franklin D. Roosevelt and the New Deal.