Theodore Lyman was born on 23 August 1833 in Waltham, Massachusetts, to Theodore Lyman, a prominent political figure who served as mayor of Boston, and Mary Elizabeth (Henderson) Lyman. After graduating from Harvard in 1855, he did graduate work in the field of zoology and studied under Louis Agassiz, the noted natural scientist. In 1855, he married Elizabeth Russell and together they had three children. Daughter Cora was born in Italy in 1862; sons Henry and Theodore were born after the Civil War. From 1861 through 1863, Lyman traveled throughout Europe collecting specimens for Harvard's new Museum of Comparative Zoology. On 3 September 1863, upon returning from Europe, Lyman joined the Union Army as a volunteer aide-de-camp to General George Meade.

Ulysses S. Grant was born on 27 April 1822 in Point Pleasant, Ohio. After graduating from West Point, he pursued a career in the military, achieving great success as a combat officer during the Mexican-American War. With the outbreak of the Civil War, Grant, who had retired from service, re-enlisted in the Army. He quickly moved up the ranks and achieved many decisive military victories for the Union Army including the battles of Forts Henry and Donelson, Chattanooga, Lookout Mountain, and Vicksburg. In March of 1864, he was commissioned to the three star rank of lieutenant general, a position previously held only by George Washington and Winfield Scott. He was subsequently appointed by President Abraham Lincoln to be general-in-chief of the Union Army. After receiving this promotion, Grant made his headquarters with the Army of the Potomac in Culpeper, Virginia.

In this letter of 18 April 1864, Lyman describes watching Grant review the 6th Corps of the Army of the Potomac. According to Lyman, Grant "was neatly dressed in the regulation uniform, with a handsome sash and sword & the three stars of a Lieutenant General on his shoulder. He is a man of a natural, severe, simplicity, in all things" (page 5). Lyman further comments that Grant, who had a reputation among his fellow soldiers as being fiercely determined to succeed in all military conflicts, sits "firmly in the saddle & looks straight ahead as if intent on getting to some particular point" (page 6).

An engaging letter writer, Lyman finds some moments of levity to share with his wife, as he recounts Grant's review of the Union troops. Upon witnessing a German artillery regiment march before Grant, Lyman writes to his wife "you would have laughed to see the stiff Prussian sergeants cropping out here & there" (page 8). He also relates that a Lieutenant Colonel Rowley, who accompanied General Grant on the review, had seemingly forgotten to attach his pants with the necessary straps for riding a horse. Lyman then commented that when a man does not possess "the happy facility of keeping down his trousers, he should make straps a part of his religion" (page 6).

At the close of the letter, Lyman tells his wife "if you see Adams, just tell him to hurry up!" and mentions that he had "electioneered" on Adams's behalf (page 9). Here, he is referring to Charles Francis Adams, Jr., Lyman's Harvard classmate and the grandson of President John Quincy Adams and son of Charles Francis Adams. Included in the Lyman papers collection at the MHS is a 12 March 1864 letter from Adams requesting Lyman's help in securing him a higher ranking position in the Union Army.

Lyman's fascination with natural sciences is evident in this epistle. In the opening paragraphs he provides Elizabeth with a detailed description of the fauna and wildlife near the army headquarters. After the war, he continued his career as a man of science. Lyman returned to Boston and taught zoology at Harvard, and in 1866, he became the first chairman of the Massachusetts Commission of Inland Fisheries, a position he held until 1883. In 1882, Lyman was elected to the United States House of Representatives as an independent and served one term, during which time he also pursued scientific interests at the Smithsonian. Lyman spent the last decade of his life as an invalid at his home in Brookline, suffering from an undiagnosed malady that resulted in his death on 9 September 1897.