He was the son of Stephen Dwinell (1745–1801) and Susanna (Olin) Dwinell. He attended Williams College, and graduated from Yale College in 1808. Then he studied law in Troy with John D. Dickinson, was admitted to the bar in 1811, and commenced practice in Cazenovia. On September 12, 1813, he married Louise Whipple, and they had nine children, among them California Assemblyman John W. Dwinelle (1816–1881).

Justin Dwinell was member of the New York State Assembly (Madison Co.) in 1820–21 and 1822. He was First Judge of the Madison County Court from 1823 to 1828.

Dwinell was elected as a Crawford Democratic-Republican to the 18th United States Congress, holding office from March 4, 1823, to March 3, 1825. He was District Attorney of Madison County from 1837 to 1845.

He was buried at the Evergreen Cemetery in Cazenovia, New York.

His last name sometimes appears as "Dwinelle," and that is how it is spelled on his gravestone.

• United States Congress. "Justin Dwinell (id: D000584)". Biographical Directory of the United States Congress.
• The New York Civil List compiled by Franklin Benjamin Hough (pages 71, 197f, 271, 361 and 376; Weed, Parsons and Co., 1858) [gives wrong listing on pg. 361]
• Reminiscences of Louise S. Dwinelle at RootsWeb
• The True Genealogy of the Dunnel and Dwinnell family of New England by Henry Gale Dunnel (pages 30 and 47)

• Justin Dwinell at Find a Grave