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 General Isaac Stevens in exploring a route for a railroad from St. Paul, Minnesota 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 during the Seminole Wars. 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 began work with the U.S. War Department in building a 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 formed a supply route to the Idaho gold mines.

On April 28, 1863 Mullan married Rebecca Williamson and they had five children. Soon after his marriage he resigned from the Army and started a ranch near Walla Walla which ultimately failed the next year. He then obtained a four-year contract to carry 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. Within a year he was forced out of business by a competitor and gave up his contract. Then settling in San Francisco, he began practicing 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 by the American Society of Civil Engineers. They 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.