After graduation, Roberts spent eight years working as a newspaperman for the Boston Post. In 1917, he enlisted in the American army for World War I, but he ended up as a lieutenant in the intelligence section of the American Expeditionary Force Siberia in the Russian Civil War instead of at the front in Europe. The contacts that he made in that role enabled him to become a European correspondent for the Saturday Evening Post after the war, where he became the first American journalist to cover the 1923 Beer Hall Putsch, Adolf Hitler's first attempt to gain power. Roberts described working for the Post's legendary editor George Horace Lorimer as follows: "I told him my ideas, which he instantly rejected or accepted.... The price to be paid for a story was never discussed, and Lorimer was always generous."

Roberts' Kennebunkport neighbor Booth Tarkington convinced him that he would never find the time to succeed as a novelist as long as he worked as a journalist, and Tarkington agreed to help by editing Roberts' early novels. Although Roberts continued to sell a few essays to the Post, his next few years were largely dedicated to historical fiction. Ultimately, Tarkington edited all of his historical novels through Oliver Wiswell, and Roberts said in his autobiography that he offered Tarkington co-writing credit on both Northwest Passage and Oliver Wiswell due to Tarkington's extensive revisions to each. Both of those novels as well as Rabble in Arms are dedicated to Tarkington.

Roberts' historical fiction often focused on rehabilitating unpopular persons and causes in American history. A key character in Arundel and Rabble in Arms is American officer and eventual traitor Benedict Arnold, with Roberts focusing on Arnold's expedition to Quebec and the Battle of Quebec in the first novel and the Battle of Valcour Island, the Saratoga campaign and the Battles of Saratoga in the second. Meanwhile, the hero of Northwest Passage was Major Robert Rogers and his company Rogers' Rangers, although Rogers fought for the British during the American Revolutionary War. Oliver Wiswell focuses on a Loyalist officer during the American Revolution and covers the entire war, from famous events such as the Siege of Boston, the Battle of Bunker Hill, the New York and New Jersey campaign through the Battle of Fort Washington, and the Franco-American alliance, to less-remembered events such as the Convention Army, the exodus to Kentucky County, the Siege of Ninety-Six, and the resettlement of the United Empire Loyalists, as well as providing a later look at both a dissolute Rogers and a frustrated Arnold among the British.

As a result of his research into the Arnold Expedition, Roberts published the nonfiction work March to Quebec: Journals of the Members of Arnold's Expedition, a compilation of various journals and letters written by participants in the march. During Roberts' research into Major Rogers, his researcher uncovered transcripts of both of Major Rogers' courts-martial (once as the accuser and once as the accused), which had been thought lost for over a century, and these were published in the second volume of a special two-volume edition of Northwest Passage. He and his wife Anna translated into English the French writer Médéric Louis Élie Moreau de Saint-Méry's account of his journey through America in the 1790s. In addition, his last published work was a brief history of the Battle of Cowpens, entitled The Battle of Cowpens, issued after his death in 1958.

One of Lorimer's last acts as editor of the Saturday Evening Post was to serialize Northwest Passage in 1936 and 1937. The success of that serialization led the book, when published, to become the second best-selling novel in America for the year 1937 and fifth for the year 1938. Oliver Wiswell also spent two years in the top 10 (1940 and 1941), and Lydia Bailey reached the top 10 in 1947.

Key historical novels by Roberts and their topics include:

• Arundel (1929) - The American Revolution through the Battle of Quebec
• The Lively Lady (1931) - War of 1812
• Rabble in Arms (1933) - Sequel to Arundel; the American Revolution through the Battles of Saratoga
• Captain Caution (1934) - War of 1812
• Northwest Passage (1937) - French and Indian War and the Carver expedition
• Oliver Wiswell (1940) - The American Revolution from a Loyalist's perspective, from the Siege of Boston to the United Empire Loyalists
• Lydia Bailey (1947) - The Haitian Revolution and the First Barbary War
• Boon Island (1955) - 1710 shipwreck on Boon Island, Maine

In 1957, two months before his death, Roberts received a Pulitzer Prize Special Citation "for his historical novels which have long contributed to the creation of greater interest in our early American history."

Three of Roberts' first books were written at least in part to promote the Florida land boom of the 1920s. They were Sun Hunting (1922), Florida Loafing (1925), and Florida (1926). Many people lost a lot of money in the bust that followed. In Roberts' subsequent books with listings of 'other books by this author', these three were usually not mentioned.

Late in his career, Roberts became acquainted with Henry Gross, a retired Maine game warden and amateur water dowser. He and Gross began a long association to use Gross' supposed dowsing abilities to find deposits of water, petroleum, uranium, and diamonds, through a corporation named Water Unlimited, Inc. Roberts documented his experiences in three nonfiction books that were popular successes but that received much criticism from the scientific community. Roberts himself joked that he should have subtitled The Seventh Sense as "Or How to Lose Friends and Alienate People."