On January 4, 1871, he preached the election sermon before the governor and newly elected officers of the Commonwealth at the Old South Church in Boston.[5] Grinnell was chaplain for the Fifth regiment Massachusetts Volunteer Militia from June 21, 1870, to May 8, 1872. He resigned from the Harvard Church in 1873, and retired from ministry altogether the following August.[3]
Grinnell then moved to Cambridge, Massachusetts and entered the Harvard Law School and graduated with a Bachelor of Laws in 1876. Following graduation, he joined the office of Chandler, Ware, and Hudson. He was admitted to the Suffolk County bar in November 1876 and opened his own office in Boston shortly after, where he practiced until 1910. Grinnell moved to Boston in July 1878 with his family. He was editor of the American Law Review from 1880 until December 1882, and again from 1906 to 1909.[1]
On July 11, 1865, Grinnell married Elizabeth Tucker Washburn. They had two sons, Charles Ewald Grinnell and Frank W. Grinnell. In 1909, Grinnell traveled with his wife to Paris and Italy, where she died in Naples. Following her death, he spent three years living in Paris and London while traveling in Germany, Russia, Egypt, the Holy Land, Greece, France, and Spain. He returned to Boston in 1913 where he lived with his son Charles. Grinnell died February 1, 1916, at his son's home in Boston and was buried at Nemasket Hill Cemetery in Middleborough, Massachusetts.[1]