The following month, as a brevet major in the U.S. Army, he was appointed to lead an expedition through the American West, in areas acquired in the Louisiana Purchase. The specific purpose of the voyage was to the find the sources of the Platte, Arkansas, and Red rivers.
In 1817, Major Long headed a military excursion up the Mississippi River to the Falls of St. Anthony near the confluence with the Minnesota River. As a result of his recommendations, the Army established Fort Snelling to guard against Indian incursions against settlers in the Upper Mississippi Valley.
Later, as an officer in the Army U.S. Topographical Engineers he led several additional expeditions into the United States borderlands with Canada, including the Upper Mississippi Valley, the Red River of the North and across the southern part of Canada.
Following his official military expeditions, Major Long spent several years helping to survey and build the Baltimore and Ohio Railroad. In 1826 he received his first patent for his work on railroad steam locomotives. Long received many more patents for locomotive design and worked with other Army engineers in planning and building the railroad.
In 1832, along with William Norris and several other business partners, he formed the American Steam Carriage Company. The business was dissolved in 1834 due to the difficulties in placing Long's locomotive designs into production.
Colonel Long received a leave of absence to work on the newly incorporated Western & Atlantic Railroad in Georgia. His yearly salary was established at $5,000, the contract was signed May 12, 1837 and he served as the chief engineer for the W&A until November 3, 1840. He arrived in north Georgia in late May and his surveying began in July and by November he had submitted an initial report which the construction followed almost exactly.
In 1838 he was appointed to a position in the newly formed U.S. topographical engineers corps. He died in Alton, Illinois in 1864.
Major Long was the leader of the first scientific exploration up the Platte River. His party included several scientists who studied the geography and natural resources of the area. Eventually, Long became one of the most prolific explorers of the period, covering 26,000 miles in five expeditions. Like most engineers, Long was college-trained, interested in searching for order in the natural world, and willing to work with the modern technology of the time. Engineers had basically two unique points of view that set them apart from the other pioneers — geographic and technological.
In July 1819, he joined Gen. Henry Atkinson's Yellowstone Expedition bound from St. Louis to the Rockies on the steamboat Western Engineer. This was the first steamboat to travel up the Missouri River into the Louisiana Purchase territory. By September 17, the steamboat arrived at Fort Lisa, a trading fort belonging to William Clark's Missouri Fur Company. It was about five miles south of Council Bluffs. Long's group built their winter quarters nearby and called it "Engineer Cantonment."
Within a month, Long returned to the east coast, and by the following May, his orders had changed. Instead of exploring the Missouri River, President James Monroe decided to have Long lead an expedition up the Platte to the mountains and back along the border with the Spanish colonies. Exploring that border was vital, since John Quincy Adams had just concluded the treaty with Spain, which drew a new U. S. border to the Pacific. On June 6, 1820, Long and 19 men traveled up the north bank of the Platte and met Pawnee and Otoe Indians. On October 14, 1820, 400 Omaha assembled at a meeting with Long; Chief Big Elk made the following speech:
After finding and naming Longs Peak and the Rocky Mountains, they journeyed down the South Platte River to the Arkansas River watershed. The expedition was then split, and Long led his group towards the Red River. They missed it, ran into hostile Indians and had to eventually eat their own horses to survive before they finally met the other part of the expedition at Fort Smith in Arkansas. Long and his party of scientists would learn much to tell the nation and have the opportunity to show the U.S. flag.
There were two key results of this expedition -- a very accurate description of Indian customs and Indian life as they existed among the Omaha, Otoes, and Pawnees and his description of the land west of the Missouri River as a "desert".
Major Long's 1823 expedition up the Minnesota River (then known as St. Peter's River), to the headwaters of the Red River of the North, down that river to Pembina and Fort Garry, and thence by canoe across British Canada to Lake Huron is sometimes confused with his initial expedition to the Red River in modern-day Texas and Oklahoma. The expedition to the Red River of the North was a separate, later appointment which completed a series of explorations conceived of by Lewis Cass and implemented by David B. Douglass, Henry Schoolcraft, and others besides Major Long. The 1823 expedition was denoted primarily as a scientific reconnaissance and an evaluation of trade possibilities, but probably had undisclosed military objectives as well, and certainly was viewed with suspicion by British authorities in Canada. This expedition for a time was joined by the Italian adventurer Giacomo Beltrami, who argued with Long and left the expedition near Fort Garry. The 1823 expedition encouraged American traders to push into the fur trade in Northern Minnesota and Dakota, and fostered the development of the Red River Trails and a colorful chapter of ox cart trade between the Red River Colony and Fort Garry via Pembina and the newly developing towns of Mendota and St. Paul.