George Dewitt was born in New Haven, New York, in 1827, the son of George and Harriet (Bartlett) Allen.
He was living with his parents and siblings in New Haven in 1850, with his occupation given as dentist. In 1853, he was called a resident of Johnstown, Pennsylvania at the time of his marriage to Mary Frances Cox.  They settled in Malden, Massachusetts, where he lived with his wife and two children in 1860 at the home of his mother-in-law.
The Civil War started in April of 1861, and in the fall of that year George D. Allen of Malden and John B. Hyde of New Bedford, Mass., organized the 5th Massachusetts Battery. They both started out as First Lieutenants. George was promoted to Captain on 25 January 1862. While serving in Virginia in June, 1862, he came down with malarial dysentery and fever caused by exposure. He took a sick leave and later resigned, being discharged for disability on 17 October 1862.
After the war, he returned to Malden, where he was a coal dealer at William Allen & Son Boiler Works. He was a member of the Masons at the Mount Vernon Lodge  and of the Knights Templar.
George died in 1900 when his body was crushed by an elevator at 7 Exchange St. in Boston.  He was rushed to the hospital but died soon after. His funeral was attended by members of the various military and civil organizations he had taken part in and 110 fellow employees of the William Allen & Sons Boiler Works. He was buried at Salem Street Cemetery in Malden.
U.S., Civil War Soldier Records and Profiles, 1861-1865 [database on-line]. Provo, UT, USA: Ancestry.com Operations Inc, 2009