Thomas Day Thacher (September 10, 1881 – November 12, 1950) was a United States district judge of the United States District Court for the Southern District of New York, the 21st Solicitor General of the United States and a Judge of the New York Court of Appeals.

Born on September 10, 1881, in Tenafly, New Jersey,^[1] Thacher was the son of Thomas Thacher, a prominent New York City, New York attorney.^[2] He attended the Taft School in Watertown, Connecticut and Phillips Academy in Andover, Massachusetts.^[2] He received a Bachelor of Arts degree in 1904 from Yale University^[1] where he was a member of Skull and Bones,^[3] and after attending Yale Law School, read law in 1906.^[1] He entered private practice in New York City from 1906 to 1907,^[1] with his father's law firm of Simpson, Thacher & Bartlett.^[2] Thacher remained with the firm and was made a partner in 1914.^[2] He was an Assistant United States Attorney for the Southern District of New York from 1907 to 1910.^[1] While in this position, Thacher was recognized for his work in prosecuting customs fraud.^[2] He returned to private practice in New York City from 1910 to 1917. In 1911 he helped found the New York Young Republican Club^[4]. ^[1] He was a Major for the American Red Cross Commission to Russia from 1917 to 1918.^[1] He again returned to private practice in New York City from 1918 to 1925.^[1]

Thacher was nominated by President Calvin Coolidge on January 9, 1925, to a seat on the United States District Court for the Southern District of New York vacated by Judge Learned Hand.^[1] He was confirmed by the United States Senate on January 19, 1925, and received his commission on January 21, 1925.^[1] His service terminated on April 10, 1930, due to his resignation.^[1]

Thacher served as the 21st Solicitor General of the United States from March 1930 to May 1933.^[1] He returned to private practice in New York City from 1933 to 1943.^[1] He helped create the movement that made possible the election of Fiorello H. La Guardia as Mayor of New York City.^[2] La Guardia appointed Thacher to serve as the leader on the commission to write a new city charter and to the city's Corporation Counsel in 1943.^[2] He was a Judge of the New York Court of Appeals from 1943 to 1948,^[1] appointed to that post by Governor of New York Thomas E. Dewey.^[2]

Thacher served as a fellow of the Yale Corporation from 1931 to 1949, President of the Association of the Bar of the City of New York from 1933 to 1935, and was a member of numerous social clubs.^[2] He first married Eunice Booth Burall, and had three children.^[2] After Eunice's death in 1943, Thacher married Eleanor M. Lloyd on July 20, 1945.^[2]

Thacher died of a coronary thrombosis on November 12, 1950, at his home in New York City.^[1][2] He was interred in Brookside Cemetery in Englewood, New Jersey.^[2]