Through his extensive time spent living in Canada, Longstreth would become enamored with the Canadian Royal Mounted Police, a subject which remained important throughout his career. He wrote many books on the subject, both fiction and non-fiction. While researching his Canadian subjects, Longstreth became an historian for the Mounted Police, even working in their office, and claimed “the distinction of being the only man outside the organization ever to have been granted access to complete records.” He lived for a time in Ottawa. His early books on the subject included The Silent Force (1927), Sons of the Mounted Police (1928), and Murder at Belly Butte and other mysteries from the records of the mounted police (1931). Later books included In Scarlet and Plain Clothes (1933), Mounty in a Jeep (1949), The Scarlet Force: The Making of the Mounted Police (1953) and The Force Carries On (1956).
Longstreth’s work about the Royal Canadian Mounted Police would launch a popular radio show on the subject in the early 1930s. Titled With Canada’s Mounted, but also known as Canada’s Mounted on the Air, the 30 minute show aired on channel NBC Blue from January 11 to April 4, 1932. The show was sponsored by the Canada Dry Bottling Company for its popular Canada Dry Ginger Ale soft drink. The lead cast on the show included Allyn Joslyn (1901-1981), a well known stage, radio, television and film actor, and Eustace Wyatt (1882-1944), a British actor and well known radio personality. Longstreth was responsible for writing “the continuity” for the show. The show would prompt a number of succeeding programs on the subject of the Mounted Police.
The show was described as “the real story of the Canadian Royal Mounted Police, written by the official historian of the group that always gets its man . . . each broadcast will be a complete story in itself, dealing with the inside history of some famous Canadian crime or criminal. The initial show tells how a famous forger was run to earth. The title is “The Case of Ernest Cashell.”
Jim Cox, author of Radio Crime Fighters: More Than 300 Programs from the Golden Age, described the show and its lasting legacy: “These dramatizations featured tales focused on Royal Canadian Mounted Police files. Instead of a single hero, as in the more prominent Challenge of the Yukon, there were two Mounties as central figures searching for lawbreakers. “The O’Brien Murders,” “The Idaho Kid,” “The Island Affair,” The Mad Trapper of the Rat River” and “Constable Whaley’s First Patrol” were among the titles of the 13-week show.
Following extensive research on the aural dramas about the Royal Canadian Mounted Police, Jack French suggested that this was the progenitor of a dozen or more series in that line (including at least a trio of narratives that were produced by the Canadian Broadcasting Corp. – Men in Scarlet [1943-48], The Queen’s Men [early 1950s], and The Quiet Force [1954-55]). The footprints of With Canada’s Mounted Police therefore loom large for it inspired a wave of successors that enthralled millions of fans young and old in two nations for nearly a quarter of a century.”
Around 1937 Longstreth “then settled down in Washington for five years; spent seven years at Concord, Massachusetts; and is now living close to his old Westtown school, which often figures in the familiar essays he has written once a month for the Christian Science Monitor” for over twenty years.” Longstreth was a member of the Author’s League (now The Authors Guild), the oldest and largest professional organization for writers.
Throughout his life Longstreth was a great admirer of the works of Henry Thoreau (1817-1862), the famous American author, naturalist and philosopher. Longstreth lived in Thoreau’s home town of Concord, Massachussetts for seven years. He served on the Library Committee for the Concord Fee Public Library and contributed to a booklet that detailed its history upon the Library’s 75th anniversary in 1948. He took frequent walks to Walden, the site of Thoreau’s famous book where “I [Thoreau] went to the woods because I wished to live deliberately, to front only the essential facts of life and see if I could not learn what they had to teach, and not, when I came to die, discover that I had not lived.”
Longstreth was living in Concord when the site of Thoreau’s cabin was discovered in 1945 by archaeologist Roland Wells Robbins (1908-1987), and was present at the excavation of the cabin’s cellar hole. Robbins wrote in his 1947 book titled Discovery at Walden that “The undersigned have witnessed the excavation of the cellar-hole of Henry David Thoreau’s Walden cabin on Labor Day, September 2, 1946 by Roland Wells Robbins who discovered the site of the chimney foundation on November 11, 1945.” The statement was signed by Roland Wells Robbins (Lincoln, Mass.), T. Morris Longstreth (Concord, Mass.), Aletta L. French (Concord, Mass.) Wallace B. Conant (Concord, Mass.), Anton Kovar (Arlington, Mass.) and Walter Harding (Secretary of the Thoreau Society, Bridgewater, Mass.). The original Thoreau site and replica cabin at Walden Pond form part of the Massachusetts State Park known as the Walden Pond State Reservation and can be visited by any fan of Thoreau.
While in Concord Longstreth participated in the informal Thoreau Reading Circle at the home of Allen and Aletta French and was a member of the Thoreau Society for many years, serving as its secretary and treasurer. The Thoreau Society was formed in 1941 and continues today as “the oldest and largest single-author society in the United States.” Longstreth wrote frequently of Thoreau in articles for the Christian Science Monitor, including “Thoreau and the Thrushes” (June 8, 1944), “On Thoreau’s Coldness” (August 25, 1944), and “Thoreau As Writing Instructor” (July 12, 1945). In 1963, at the age of 77, Longstreth authored Henry Thoreau, American Rebel. In the introduction he wrote of Thoreau’s profound influence on his life:
“Best of all, books give us their creators, the wise and great and funny and lasting people who saw deeper into human beings and farther into spiritual greatness than most of us. Anybody who can read soon starts acquiring friends of this sort, and quite by accident I fell in step with Henry Thoreau when I was thirteen. I was as wild-feeling as he, as crazy about weather and the woods and rivers. I liked to laugh just as much. This happened in 1899, and we are better friends than ever today.
This is because I wanted to be I, and he wanted to be completely himself. It is because I wanted more life, and his suggestions as to how to get it were simpler, more practical and more exciting than any I heard elsewhere. It is because I wanted to rich and lazy, to live in the country and have everything the city offered, to stay young however old I had to be, to enjoy every moment in spite of everything. The evening I first looked into Thoreau’s Journals in the Westtown School library, was one of the great beginning moments of my whole experience. And I am still turning to them for pleasure and laughs, for aspects of nature I have overlooked, for good practical common sense, and above all, for my friend Henry Thoreau. “The bluebird carries the sky on his back,” he said, and Henry Thoreau carries the fire of life in his sentences. The more of them you have made your own, the richer and wiser you will be.”
The 1950 book Current Biography offered a few more personal details about Longstreth: “In view of the fact that Longstreth does not consider himself photogenic, and thinks press photographs misleading anyway, no picture of him appears here. He is of medium height, with fair hair and blue-gray eyes. He is a member of the Society of Friends, a bachelor, and a Republican in politics (“for default of better”). Skiing and canoeing were his favorite sports, and he still enjoys music and the outdoors. Besides his nearly thirty books [later over forty], he was written hundreds of magazine articles. His most widely quoted sentence is, “As youth goes, so does the nation.” And he has read Huckleberry Finn as often as The Virginian and the poems of Robert Frost – as well as Hamlet and “the great pieces in the Bible.”
Longstreth lived in Westtown, Pennsylvania from 1949 until his death. Longstreth died at Wynnewood, Pennsylvania on December 21, 1975.
 “Radio News.” The Daily Star. January 11, 1932. Page 8.
 “With Canada’s Mounted Is New Air Story Series.” Brooklyn Daily Eagle. January 10, 1932. Page C 12.
 Cox, Jim. Radio Crime Fighters: More than 300 Programs from the Golden Age. North Carolina: McFarland & Company, Inc., 2002. Page 277.
 Rothe, Anna. Current Biography. New York: H. W. Wilson Company, 1951. Pages 349-350.
 Robbins, Roland Wells. Discovery at Walden. Stoneham, Mass.: George R. Barnstead & Son, 1947. Page 51.
 Longstreth, T. Morris. Henry Thoreau, American Rebel. New York: Dodd, Mead & Company, 1963. Pages IX-X.
 Rothe, Anna. Current Biography. New York: H. W. Wilson Company, 1951. Pages 349-350.