John Mandt Nelson (October 10, 1870 – January 29, 1955) was a U.S. Representative from Wisconsin.
Born in Burke, Wisconsin, Nelson attended the public schools and was graduated from the University of Wisconsin–Madison in 1892. Superintendent of schools in Dane County in 1892 and 1894. Bookkeeper in the office of the secretary of state 1894–1897. He was editor of The State, published in Madison, Wisconsin, in 1897 and 1898. Correspondent in the State treasury 1898–1902. He was graduated from the law department of the University of Wisconsin–Madison in 1896, and pursued a postgraduate course 1901–1903.
Nelson was elected as a Republican to the Fifty-ninth Congress to fill the vacancy caused by the death of Henry C. Adams. He was reelected to the Sixtieth and to the five succeeding Congresses and served from September 4, 1906, to March 3, 1919. He was an unsuccessful candidate for renomination in 1918.
Nelson was elected to the Sixty-seventh and to the five succeeding Congresses (March 4, 1921 – March 3, 1933). He served as chairman of the Committee on Elections No. 2 (Sixty-eighth Congress). Committee on Invalid Pensions (Seventy-first Congress). He was an unsuccessful candidate for renomination in 1932 to the Seventy-third Congress. He retired from business and political activities. He died in Madison, Wisconsin, January 29, 1955. He was interred in Forest Hill Cemetery.