Cogswell was born in Bradford, Massachusetts, to George Cogswell and Abigail (Parker) Cogswell. Cogswell's father was a well respected surgeon and one of the founders of the Massachusetts Republican party. Abigail's mother died when he was about 7 years old.
Cogswell attended Atkinson Academy in Atkinson, New Hampshire, Kimball Union Academy in Meriden, New Hampshire, Phillips Academy in Andover, Massachusetts, and Dartmouth College in Hanover, New Hampshire.
Cogswell entered Dartmouth in 1855, he left Dartmouth soon after and from 1856 to 1857 he went on a voyage around the world, spending two years before the mast as a sailor. When Cogswell returned from his voyage he entered Harvard Law School.
On September 8, 1860 Cogswell was admitted to the bar in Essex County, Massachusetts. He worked for a while in the office of Attorney William D. Northend, and in April 1861 Cogswell opened his own office in Salem, Massachusetts.
Cogswell was a private in the Second Corps of Cadets, a militia organization of the Commonwealth of Massachusetts. Cogswell served in the Second Corps of Cadets during the winter of 1860-1861.
On April 19, 1861, word reached Salem that the Sixth Massachusetts had been attacked in Baltimore, Maryland while on its way to defend Washington, D.C. Cogswell turned his office into a recruiting station and in 24 hours raised a full company, the first company in the country recruited for the war. This became Company C of the Second Massachusetts Volunteers with Cogswell as Captain in command.
Cogswell was commissioned a captain in the Second Regiment, Massachusetts Volunteer Infantry, May 11, 1861. He was promoted to lieutenant colonel on October 23, 1862, and to colonel on June 25, 1863. He was brevetted Brigadier General of Volunteers on December 15, 1864, and mustered out on July 24, 1865.
Return to the practice of law
After the Civil War Cogswell resumed the practice of his profession.
He served as mayor of Salem 1867-1869, 1873, and 1874. He served as member of the Massachusetts House of Representatives 1870, 1871, and 1881-1883. He served in the Massachusetts State Senate in 1885 and 1886. He served as delegate to the Republican National Convention in 1892.
Cogswell was elected as a Republican to the 50th United States Congress and to the four succeeding congresses and served from March 4, 1887, until his death in Washington, D.C., May 22, 1895. He was interred in Harmony Grove Cemetery, Salem, Massachusetts.
- List of American Civil War generals
- List of Massachusetts generals in the American Civil War
- Memorial Addresses on the Life and Character of William Cogswell (late a Representative from Massachusetts): Delivered in the House of Representatives and Senate, Fifty-fourth Congress, First and Second Sessions (1897).
- William Cogswell at the Biographical Directory of the United States Congress Retrieved on 2008-02-12
- William Cogswell as a member of the 50th Congress Massachusetts Delegation.