John Mullan was born July 31, 1830 in Norfolk, Virginia, the first of ten children. In 1848, he was admitted to the United States Military Academy. In 1852 he graduated and was assigned first to the topographical engineers and then to the artillery. In 1853, he joined Gen. Isaac Stevens in exploring a route for a railroad from St. Paul to the Pacific. He was later sent to examine the western mountain passes. He explored the Rocky Mountains southward to Fort Hall on the Snake River and north to Canada, discovering Mullan Pass.

Promoted to first lieutenant in 1855, Mullan was recalled to active military duty and spent two years in the South fighting the Seminoles. Meanwhile, Congress authorized the construction of a military road from Fort Benton to Walla Walla to connect navigation on the Missouri with that on the Columbia.

In the spring of 1859, Mullan, directed by the U.S. War Department, began the tedious task of building the road with over 200 soldiers and civilians. It reached Fort Benton, Montana on August 1, 1860. This created a boom for the emerging town of Walla Walla, as it created a supply route to the Idaho gold mines

On April 28, 1863 Mullan married Rebecca Williamson and they eventually had five children. Soon after his marriage he resigned from the Army and started a huge ranch near Walla Walla which failed the next year. He then obtained a four-year contract to carry the mail from Chico, California to Ruby City, Idaho, a distance of 600 miles, at the rate of $75,000 a year, and attempted to establish an express business, but within a year was forced out of business by a competitor and gave up his contract. Settling in San Francisco, he began the practice of law and was quite successful. He moved to Washington, D.C. in 1878, and there continued his legal work until failing health forced his retirement. He died in Washington.

On October 28, 1978 Mullan's contribution to the settling of the Inland northwest was recognized when the American Society of Civil Engineers dedicated the Mullan Road as a National Historic Engineering Landmark by placing a plaque commemorating the accomplishment at the top of Fourth of July Pass in Idaho near the famed Mullan Tree.