Nathaniel H. McLean was born in Piqua, Ohio, on November 27, 1827, to William McLean, a congressman from Ohio, and his wife, Sarah Fox. He spent most of his childhood in Cincinnati, Ohio, and graduated from West Point in 1848. He rose up the ranks in the army, being appointed second lieutenant of infantry in 1848, brevet captain assistant adjutant general in 1861, and major assistant adjutant general the following year. He was stationed with his regiment in California for much of the 1850s, and from late 1861 to 1863, served the Departments of Ohio, Mississippi, and Missouri. In June 1857, he married Sarah "Sallie" Kilbreth, and they had four children, only two of whom, Mary and Carolyn, lived to maturity.
In July 1863, McLean was chosen to investigate the assistant quartermaster of volunteers, Captain Francis W. Hurtt. Hurtt, who was a prominent Cincinnati businessman, had allegedly defrauded the government by using public money for private purchases, speculating on rations sold to troops, and abusing his position. McLean reported his findings in September 1863. On November 23 and 24, General Ambrose Burnside issued Special Orders Nos. 436 and 437 to convene a court martial to try Hurtt. However, before McLean could reveal his conclusions, the War Department revoked the order for a court martial, and removed Major McLean from his post in Cincinnati to one in distant Vancouver, Washington Territory. Newspapers, such as the Cincinnati Gazette, speculated that Secretary Stanton had taken this action to avoid a large-scale embarrassment for the War Department. Hurtt was tried and convicted by court martial in 1864, but McLean's attempts at redress were futile, and he resigned June 24, 1864. Congress took up his case ten years later, and in March 1875, the Committee on Military Affairs agreed that he had been treated unjustly by the War Department, and appointed him to the rank of lieutenant colonel. McLean died July 5, 1884, in Cincinnati, Ohio.
McLean's widow, Sarah, sued for the payment to which her husband would have been entitled, and won back pay and emoluments in a case decided by the Supreme Court on December 23, 1912.