Harrison H. Cook was born around 1840, possibly in Illinois. By the early 1860s, he was living in Lisbon, Illinois, and working as a farmer. He enlisted in the Union army on July 18, 1862, and mustered in as a private in Company E of the 91st Illinois Infantry on September 8. According to his service records, he was 5 feet, 11½ inches tall, with light hair and gray eyes.
The regiment spent the rest of 1862 guarding the Louisville and Nashville Railroad. Confederate forces captured them on December 28, 1862, but they were paroled soon afterward, and they spent the next five months as paroled prisoners of war at Benton Barracks, Missouri. They were exchanged on June 5, 1863, and the army moved them to Vicksburg the following month. They spent the following two years stationed along the Gulf Coast. In January 1865, he wrote, "here I am in my 25th year and have only got about two hundred and fifty dollars and I think that I could have done better if I had staid at home but some one has got to go and help fight the battle of Freedom." He mustered out on July 12, 1865. He applied for a federal pension in November 1870 and eventually secured one. He eventually moved to Hubbard, Ohio, and he died there on June 23, 1918.