Newton enlisted in the New Hampshire National Guard shortly after the beginning of Civil War hostilities. In October, 1861 he enlisted in the regular army, joining the U.S. Engineer Corps. Advancing from private to corporal in October, 1862 he saw extensive service and was engaged in the construction of pontoon bridges and fortifications in Eastern Virginia. In April, 1864, he claims in his letters, that he was commissioned Captain of Company C, War Department Rifles and was assigned to duty at the White House, guarding President Lincoln. Often known for inflating the truth, this claim was later proved to be bluster. During the war he kept a diary in which he made many sketches. Letters, written frequently to his older brother, Vaola John, have been recorded for posterity. His artistic ability was demonstrated during a trip home when he married Hannah Maria Barker. He refused an offer of promotion and began a career as a portrait artist. An attempt at living in the wild was short lived after buying and selling land in Minnesota, property that would later be occupied by Pillsbury Flour Co. His young wife, Hannah was awakened one morning by noises in the distant field. She witnessed a small hunting party of Indians which ultimately resulting in her adamant determination to leave the wilds of Minnesota for the safety of New England. Within the month, they returned to New Hampshire.

His greatest achievement was the charcoal sketch he did of Ulysses S. Grant. The remarkable sketch together with a letter of introduction from the president served him well. He painted the portraits of many prominent persons, both in this country and those he executed in Europe during his two trips there. During one of these trips, he stayed in the Hartshorn House in the village of Hartshorn, Derbyshire. In was in this house that Sir Walter Scott wrote the book, Ivanhoe. He reflected on the origins of the Hartshorn family and did a brief genealogical study.

Many of the portraits that he painted presently hang in libraries and museums in New England. Several are on display at the Baker Library at Dartmouth College. Some of his commissions were paid for with merchandise in kind. One such painting made of the president of the International Silver Company was paid for with a complete silver service. A serving pitcher from this service is presently in the possession of this compiler, as well as letters of commendation, including one signed by President Ulysses S. Grant.

Newton Hartshorn was satisfied to live off the laurels of his artist's reputation and did little else to sustain himself or his family in his later years. He became a gentleman farmer, retiring to Sandisfield, in the foothills of western Massachusetts. During the autumn harvest, he could be seen with a horse-drawn wagon, taking a load of home-grown apples to market in Boston, a distance of over 125 miles. He suffered from stomach cancer and spent his remaining months with daughter, Caroline, in Springfield, Massachusetts. He died on 28 Mar 1922 at the age of 80 in Springfield, Hampden County, Massachusetts. Newton was buried in South Sandisfield Cemetery, Sandisfield, Berkshire County, Massachusetts..