Powell's loyalties remained with the Union and the cause of abolishing slavery. On May 8, 1861, he enlisted at Hennepin, Illinois, as a private in the 20th Illinois Infantry. He was described as "age 27, height 5′ ″ tall, light complected, gray eyes, auburn hair, occupation teacher." He was elected sergeant-major of the regiment, and when the 20th Illinois was mustered into the Federal service a month later, Powell was commissioned a second lieutenant. He enlisted in the Union Army as a cartographer, topographer and military engineer.