Henry Clay Metzger was born on February 2, 1839, in Hanover, Pennsylvania, to Frederick E. Metzger and Eliza A. Emich. His father was a merchant and self-described "Gentleman" who owned $14,000 in real estate by 1860. Metzger attended school in York County, and he enlisted in the Union army in April 1861. He mustered in as a private in Company G of the 16th Pennsylvania Infantry on April 25. After completing their three-month period of enlistment, the men mustered out on July 30, 1861. Robert E. Lee's Army of Northern Virginia advanced northward in the summer of 1863, prompting Metzger to enlist in the 26th Pennsylvania Infantry Militia on June 17, 1863. He was honorably discharged on July 30 after Lee's army retreated into Virginia. He enlisted in the Union army again on March 4, 1864, and mustered in as a private in Company G of the 184th Pennsylvania Infantry. He became a bugler soon afterward, and he was honorably discharged on July 14, 1865.
Metzger returned to Pennsylvania after the war, and he married Emma Cora Myley. They had at least four children: Frederick, born in October 1868; Eugene, born around 1870; Grace, born in December 1872; and Maurice, born in February 1876. He died in Hanover on December 2, 1877.