After the American Civil War, Rev. Reed remarried, to Mrs. Mary C. Byrd Kyle of Harrisonburg, Virginia, with whom he would have a daughter. Young Walter enrolled at the University of Virginia. After two years, Reed completed the M.D. degree in 1869, two months before he turned 18. He was the youngest-ever recipient of an M.D. from the university.

Reed then enrolled at the New York University’s Bellevue Hospital Medical College in Manhattan, New York, where he obtained a second M.D. in 1870, as his brother Christopher attempted to set up a legal practice. After interning at several New York City hospitals, Walter Reed worked for the New York Board of Health until 1875.

Finding his youth limited his influence, and dissatisfied with urban life, Reed joined the U.S. Army Medical Corps. This allowed him both professional opportunities and modest financial security to establish and support a family. After Reed passed a grueling thirty-hour examination in 1875, the army medical corps enlisted him as an assistant surgeon. By this time, two of his brothers were working in Kansas, and Walter soon was assigned postings in the American West. Over the next sixteen years, the Army assigned the career officer to different outposts, where he was responsible not only for American military and their dependents, but also various aboriginal American tribes, at one point looking after several hundred Apaches, including Geronimo. Reed noticed the devastation epidemics could wreak and maintained his concerns about sanitary conditions. During one of his last tours, he completed advanced coursework in pathology and bacteriology in the Johns Hopkins University Hospital Pathology Laboratory.

He married Emily Blackwell Lawrence (1856–1950) of North Carolina on April 26, 1876 and took her West with him. Later, Emily would give birth to a son, Walter Lawrence Reed (1877–1956) and a daughter, Emily Lawrence Reed (1883–1964), and the couple also adopted an aboriginal American girl named Susie, while posted at frontier camps.

In 1893, Reed joined the faculty of the George Washington University School of Medicine and the newly opened Army Medical School in Washington, D.C., where he held the professorship of Bacteriology and Clinical Microscopy. In addition to his teaching responsibilities, he actively pursued medical research projects and served as the curator of the Army Medical Museum, which later became the National Museum of Health and Medicine (NMHM). These positions also allowed Reed to break free from the fringes of the medical world.

In 1896, Reed first distinguished himself as a medical investigator. He proved that yellow fever among enlisted men stationed near the Potomac River was not a result of drinking the river water. He showed officials that the enlisted men who got yellow fever had a habit of taking trails through the local swampy woods at night. Their yellow fever-free fellow officers did not do so. Reed also proved that the local civilians drinking from the Potomac River had no relation to the incidence of the disease.

Reed traveled to Cuba to study diseases in U.S. Army encampments there during the Spanish–American War. Appointed chairman of a panel formed in 1898 to investigate an epidemic of typhoid fever, Reed and his colleagues showed that contact with fecal matter and food or drink contaminated by flies caused that epidemic. Yellow fever also became a problem for the Army during this time, felling thousands of soldiers in Cuba.

In May 1900, Major Reed returned to Cuba when he was appointed head of the Army board charged by Surgeon General George Miller Sternberg to examine tropical diseases, including yellow fever. Sternberg was one of the founders of bacteriology during this time of great advances in medicine due to widespread acceptance of Louis Pasteur’s germ theory of disease, as well as the methods of studying bacteria developed by Robert Koch. During Reed’s tenure with the U.S. Army Yellow Fever Commission in Cuba, the board both confirmed the transmission by mosquitoes and disproved the common belief that yellow fever could be transmitted by clothing and bedding soiled by the body fluids and excrement of yellow fever sufferers – articles known as fomites. The board conducted many of its dramatic series of experiments at Camp Lazear, named in November 1900 for Reed’s assistant and friend Jesse William Lazear, who had died two months earlier of yellow fever while on this assignment.

Reed’s breakthrough in yellow fever research is widely considered a milestone in biomedicine, opening new vistas of research and humanitarianism. It was largely an extension of Carlos J. Finlay’s work, carried out during the 1870s in Cuba, which finally came to prominence in 1900. Finlay was the first to theorize, in 1881, that a mosquito was a carrier, now known as a disease vector, of the organism causing yellow fever: a mosquito that bites a victim of the disease could subsequently bite and thereby infect a healthy person. He presented this theory at the 1881 International Sanitary Conference, where it was well-received. A year later Finlay identified a mosquito of the genus Aedes as the organism transmitting yellow fever. His theory was followed by the recommendation to control the mosquito population as a way to control the spread of the disease.

After Reed returned from Cuba in 1901, he continued to speak and publish on yellow fever. He received honorary degrees from Harvard and the University of Michigan in recognition of his seminal work.

In November 1902, Reed’s appendix ruptured. He died on November 22, 1902, of the resulting peritonitis, at age 51. He is buried in Arlington National Cemetery.

In 1912, he posthumously received what would come to be known as the Walter Reed Medal in recognition of his work to combat yellow fever. A tropical medicine course is also named after him, Walter Reed Tropical Medicine Course. The National Library of Medicine in Bethesda, Maryland holds a collection of his papers regarding typhoid fever studies. Philip Showalter Hench, a Nobel Prize winner for Physiology or Medicine in 1950, maintained a long interest in Walter Reed and yellow fever. His collection of thousands of items—documents, photographs, and artifacts—is at the University of Virginia in the Philip S. Hench Walter Reed Yellow Fever Collection. More than 7,500 of these items, including several hundred letters written by Reed himself, are accessible online at the web exhibit devoted to this Collection.

Reed’s name is featured on the frieze of the London School of Hygiene & Tropical Medicine. Twenty-three names of public health and tropical medicine pioneers were originally chosen to be displayed on the School building in Keppel Street when it was constructed in 1926. In 2019, the LSHTM received permission to alter the historic building by adding the names of three eminent women.

John Miltern portrayed Reed in the 1934 Broadway play, Yellow Jack, written by Pulitzer Prize winner Sidney Howard, in collaboration with Paul de Kuif . Harcourt Brace and Co. published the play in book form, titled Yellow Jack : A History, in 1934. Lewis Stone took the part in Metro-Goldwyn-Mayer’s 1938 film adaptation of the play, Yellow Jack. The play and screenplay were adapted for television in episodes (both titled “Yellow Jack”) of Celanese Theatre (1952) and of Producers’ Showcase (1955). In the latter, Reed was portrayed by Broderick Crawford.

Jeffrey Hunter played Reed in a 1962 episode of the anthology show Death Valley Days, titled “Suzie”. In 2006, PBS’s American Experience television series broadcast, “The Great Fever”, a program exploring Reed’s yellow fever campaign. The PBS website contains a great deal of additional information, including links to primary sources.