Upon the reorganization of the Army in 1816, he was appointed a Major on 16 April and assigned to the Southern Division under Maj. Gen. Andrew Jackson as a topographical engineer.

As a result of his recommendations, the Army established Fort Snelling to guard against Native American incursions against settlers in the Upper Mississippi Valley.

Long recorded his experiences of the expedition in a journal, which was first published as Voyage in a Six-oared Skiff to the Falls of St. Anthony, by the Minnesota Historical Society in 1860.

[5] Following his official military expeditions, Major Long spent several years on detached duty as a consulting engineer with various railroads.

However a then provision in the Maine Constitution which prohibited public loans for purposes such as building railroads as well as the financial panic of 1837 intervened to kill the project.

[7] Colonel Long received a leave of absence to work on the newly incorporated Western & Atlantic Railroad in Georgia.

[8] 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.

Like most of their officers Major Long remained loyal to the Federal government during the Civil War, and he became Colonel of the Corps in 1861[10] until its merger back into the U.S.

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.

This was planned to explore the upper Missouri, and Long spent the autumn designing the construction of an experimental steamboat for the venture, Western Engineer.

Major Long was the leader of the first scientific exploration up the Platte, which planned to study the geography and natural resources of the area.

They missed it, ran into hostile Native Americans and had to eventually eat their own horses to survive before they finally met the other part of the expedition at Fort Smith (now a city on the western border of the state of Arkansas).

In his report of the 1820 expedition, Long wrote that the Plains from Nebraska to Oklahoma were "unfit for cultivation and of course uninhabitable by a people depending upon agriculture."

Long felt the area labeled the "Great Desert" would be better suited as a buffer against the Spanish, British, and Russians, who shared the continent with the United States.

There was little timber for houses or fuel, minimal surface water, sandy soil, hard winters, vast herds of bison, resident Native Americans, and no easy means of communication.

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 Bates Douglass, Henry Schoolcraft, and others besides Major Long.

The 1823 expedition was denoted primarily as a scientific reconnaissance (one of its members was geologist William H. Keating)[11] 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.

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.

After a completing a survey Long had objections to building at Napoleon because of the tendency for flooding and likelihood that the town would face destruction in the future thanks to the unwieldy Mississippi and Arkansas Rivers.