He was still living at home with his mother in Gillespie township, at the time of the commencement of the war of the rebellion. He was then in his twenty-third year. Promptly on the first call for troops in April, 1861, he enlisted, and was mustered in Co. H of the 9th Illinois regiment. During the summer of 1861 the regiment was stationed at Cairo, and while laying there he was taken sick, sent to the hospital, and in the fall discharged from the service by reason of disability. He came home almost a complete physical wreck, the change in his appearance being so great that he was recognized with difficulty by some of his most intimate acquaintances. His recovery was rapid during the succeeding winter, and on the 15th of July, 1862, he re-enlisted in the army for three years. He was mustered in as sergeant-major of the 97th Illinois regiment, and was detailed to drill recruits at Springfield, where he remained till November, 1862, when the regiment was ordered to Kentucky and attached to the Division commanded by Gen. A. J. Smith. In December the 97th moved to Memphis, and was made a part of the 13th Army Corps. The regiment took part in the campaign against Vicksburg, forming a part of the right wing of the army of the Tennessee under Gen. Sherman, which attacked Vicksburg by way of Yazoo City. During this attack he was placed in command of Co. C, which he had become destitute of officers. The regiment next proceeded to Post Arkansas, on the Arkansas river, where Gen. McClenard succeeded to command of the 13th Corps. At the fight at Post Arkansas, on the 11th of January, 1863, Capt. Pentzer commanded Co. C, and was recommended for promotion on the battle field for bravery displayed during the engagement. The 97th Illinois was next stationed at Young's Point, Louisiana, where the men were engaged in digging the canal intended by Gen. Grant to change the course of the Mississippi, and cut off Vicksburg from river communication. Capt. Pentzer, in charge of a corps of men, was employed for some time in assisting in laying out the canal. The regiment subsequently crossed to the east side of the Mississippi at Grand Gulf, and took part in the battle of Port Gibson, the first engagement on the east side of the Mississippi below Vicksburg. Gen. Grant then took personal command of the forces against Vicksburg, and Capt. Pentzer took part in all the important movements of the campaign which culminated in the capture of Vicksburg, on the 4th of July, 1863, including the battle of Champion Hill, and other important engagements. In June, 1863, he received his commission as captain, which dated back to his promotion the preceding February.

After the surrender of Vicksburg his regiment formed part of the force with which Sherman drove back the Confederate Gen. Joe Jackson, and then returned to Vicksburg. After a short furlough he rejoined his company at Carrollton, Louisiana, the regiment having been placed under Gen. Banks' command. He took part in the Bayou Teche expedition, and was then ordered to New Orleans. While en route from New Iberia to New Orleans the train bearing the regiment collided with another train coming around a curve. This accident occurred at night, and was unusually fatal in its consequences, occasioning the death or permanent disability of about a hundred men belonging to the regiment. The 97th Illinois was assigned to post duty at New Orleans, and for seven months Capt. Pentzer had charge of rebel prisoners at the Custom House. From eight hundred and eighty-four active men the regiment had dwindled down to two hundred and eighty, and while in New Orleans the ranks were filled up by recruits. From July to November, 1864, the regiment was engaged in river patrol duty at Morganza Bend, two hundred miles above New Orleans, and was kept constantly alert scouting the country for the guerrillas who infested the river banks. While there Capt. Pentzer saw the hardest service he experienced while in the army. His command was subsequently sent to Dauphin Island in Mobile Bay, and thence to the mouth of Pascagoula river. In January, 1865, they reached Barancas, twelve miles below Pensacola, on the coast of Florida; the following March went into camp at Pensacola; and subsequently marched across Florida, cutting their way through timber and swamps, and constructing corduroy roads, miles in length, to Mobile Bay. He took part in the capture of Fort Blakely, on the 9th of April, 1865, the last battle of any prominence which occurred during the war. he had the honor of receiving in person the surrender of Gen. F. M. Cockrell (now United States Senator from Missouri), in command of the Confederate forces. His command then proceeded up the Alabama river to Selma, and returned to Mobile, and took part in the capture of a small railroad station. This was two or three weeks after the fall of Richmond, but news of the end of the war had not yet reached that part of the army. June, 1865, he was detailed for service on a general court martial, which sat for a month in the Custom House at Mobile, and afterward during the month of July at Galveston, Texas, to which point his regiment had been ordered. In the early part of August he was mustered out at Galveston and discharged at Springfield, in this state.

Returning to Macoupin county he engaged in farming. June, 1870, he married Miss Mary F. Adams, born in Ohio, daughter of John Adams, who settled in Gillespie township in 1847. The anti-slavery sentiments of Capt. Pentzer's father had been one reason of the removal of the family from Missouri to Illinois. Naturally, therefore, on growing up he became a republican, and voted for Lincoln for President in 1860. His service in the army is sufficient indication of his patriotism during the war of the Rebellion. Two younger brothers also served in the Union army. In 1868 his views differed from the policy of the republican party as represented by the Grant administration, and believing that the best intervals of the country demanded a change, he became a democrat. In 1873 he was elected assessor of Gillespie township. In 1874 he was chosen supervisor, and has since been re-elected each year to that position. He has been known as one of the most active members of the Board, for six years has been chairman of the Finance Committee, and has served on other important committees. His course regarding the Court House and other difficult questions with which the Board has had to deal seems to have met with the cordial approval of the people of the county. He opposed the payment or recognition of the old Court House debt, but advocated a settlement of the claims against the county on a just and equitable basis and then their prompt payment. He was appointed a committee to visit eastern cities and confer with the bond holders as to the basis of a proposed settlement and was influential in securing the passage of the act through the legislature of 1877 by which the county was enabled to fund the bonds on such a basis as the county and its creditors could agree upon. His efforts have been successful, and he has the satisfaction of known that the exertions of the Board have left matters in a better shape for an equitable settlement than has been the case in all the history of these transactions. He has two children, Chatty F. and Henry Earl Pentzer. For four years he has been a member of the democratic central committee.