Benjamin Browne Foster was born on November 23, 1831, in Orono, Maine, to Cony and Caroline Foster. His father was a merchant who owned $2,500 of real estate by 1850. He grew up and attended school in Orono, Maine, and by 1850, he was working as a clerk. He enrolled at Bowdoin College in the early 1850s and earned a degree several years later. By 1860, he was working as a lawyer in Castine, Maine.
On November 2, 1861, he received a commission as a 1st lieutenant in Company I of the 11th Maine Infantry. The regiment took part in the Peninsula campaign, Battle of Seven Pines, and the Seven Days’ Battles. On October 7, 1862, he was promoted to major and assistant adjutant general on the staff of General John J. Peck. He resigned on September 24, 1864, and he later received a brevet promotion to colonel.
He settled in Norfolk, Virginia, after the war and resumed his work as a lawyer. He married a woman named Sarah, and they had at least five children: Howell, born around 1865; Winthrop, born around 1867; Louisa, born around 1869; Gibson, born around 1872; and Raymond, born around 1874. By 1870, they owned $4,000 of real estate and $10,000 of personal property, and they employed at least three Black domestic servants.
They moved to Brooklyn, New York, in the 1870s. He applied for a federal pension in March 1894 and eventually secured one. His wife died on February 14, 1897, and he passed away on May 16, 1903.