Josiah Gregg, frontiersman, the son of Harmon and Susannah (Smelser) Gregg, was born in Overton County, Tennessee, on July 19, 1806, the youngest son in a family of seven children. He grew up in Illinois and in Howard County, Missouri, where he received most of his formal schooling. Sickly, shy, and studious, he taught school near Liberty, Missouri, in 1824, and the following year moved with his family to Independence, where he studied law and surveying before his health broke in 1830 under the consumption and chronic dyspepsia from which he suffered. On the advice of his doctors Gregg made his first trip west in the spring of 1831, across the plains to Santa Fe with a merchant caravan. The outdoor life quickly improved his health, and Gregg was hired as a bookkeeper by Jesse Sutton, one of the merchants.
He returned to Missouri in the fall of 1833, then set out again the following spring as Sutton's business partner and wagonmaster of the caravan. Before the end of the decade he traversed the plains four times and ventured over what is sometimes called the Chihuahua Trail into the interior of Mexico. He mastered the Spanish language and by 1840 had emerged as a fairly successful merchant.
During the late 1830s, when the "Pastry War" momentarily halted the flow of imported items from Veracruz to northern Mexico, Gregg sought to take advantage of the situation by freighting goods overland from the United States for sale in Chihuahua. After making one round trip in 1839 he sought out a shorter, more southerly route from Santa Fe to the Mississippi valley. He decided to follow the south side of the Canadian River and left Santa Fe on February 25, 1840, with forty-seven men, twenty-eight wagons, two canons, 200 mules, and 300 sheep and goats. As it crossed the Panhandle in March, Gregg's expedition fought off an attack by a band of Pawnees near Trujillo Creek in what is now Oldham County, and a few days later had to endure a blue norther that scattered most of the sheep and goats permanently across the Llano Estacado. In his journal Gregg carefully noted the terrain, the abundance of game, and the unpredictable Panhandle weather. The expedition crossed the 100th Meridian on March 23 and moved on through Indian Territory to Fort Smith and Van Buren, Arkansas.
This venture marked Gregg's last return from New Mexico, but a few months later he toured Indian Territory as far west as Cache Creek in the Comanche country. In the summer of 1841 and again in the winter of 1841–42 he traveled through Texas, up the Red River valley, and later from Galveston to Austin and by way of Nacogdoches to Arkansas. He sold mules to the Republic of Texas for a profit and took note of Texas geology, trees, prevalent attitudes, and politics. With his earnings he settled for a time at Van Buren and opened a general store in partnership with his brother John and George Pickett.
At the same time, Gregg began compiling his travel notes into a readable manuscript and in the summer of 1843 went to New York to secure a publisher. His Commerce of the Prairies, which came out in two volumes in 1844, was an immediate success. It went through two new editions in 1845, later a fourth and fifth edition, and in 1857 appeared in a sixth edition under a different title. The book, which also had a large sale in England and was translated into French and German, remains the cornerstone for all studies of the Santa Fe Trail. In 1845 Gregg produced the most complete and reliable map of the southern plains at that time; his notes were used by later mapmakers who explored the Canadian, and his speculations regarding the source of the Red River prompted the expedition of Randolph B. Marcy and George B. McClellan in 1852.
In the fall of 1845 Gregg enrolled at the University of Louisville medical school, where he studied for two semesters. The following spring he joined the wagon caravan of Samuel C. Owens at Independence. However, news of the outbreak of the Mexican War in May 1846 prompted him to leave the Owens train and join Gen. John E. Wool's Arkansas Volunteers as an interpreter and unofficial correspondent. In that role he submitted several articles, including an eyewitness account of the battle of Buena Vista, to western newspapers and collected cartographical data for the War Department.
After a proposed business scheme with Samuel Magoffin fell through, Gregg practiced medicine with Dr. G. M. Prevost in Saltillo until the spring of 1848. Having become acquainted with the German naturalist Frederick A. Wislizenus, he joined a botanical expedition to western Mexico and California, during which he corresponded with and sent specimens to the eminent botanist George Engelman in St. Louis. Subsequently, the American Botanical Society added the Latin name greggi in his honor to twenty-three species of plants.
After the end of the expedition in the summer of 1849 Gregg sailed for San Francisco, left his field notes with Jesse Sutton, who had settled there, and joined in the gold rush to the mother-lode country. In October he led an exploring party through the uncharted redwood forests and discovered Humboldt Bay. The party named the Van Duzen and Eel rivers and other familiar landmarks in that area. On the return trip to San Francisco the group split into two, Gregg's division turning inland to Clear Lake. Exhausted from vigorous travel, near-starvation, and continuous exposure to severe weather, Gregg died on February 25, 1850, as a result of a fall from his horse, and was buried near the lake.