Lord was born in Baltimore, Maryland to John Walterhouse and Henrietta Hoffman. His father was a lawyer who died when Walter was just three years old. His grandfather, Richard Curzon Hoffman, was president of the Baltimore Steam Packet Company steamship firm in the 1890s.

Following high school at Baltimore's Gilman School, he studied history at Princeton University, graduating in 1939. Lord then enrolled at Yale Law School, interrupting his studies to join the Army after the attack on Pearl Harbor. During World War II, he was assigned to the Office of Strategic Services as a code clerk in London in 1942. He was the agency's secretariat when the war ended in 1945. Afterwards, Lord returned to Yale where he earned a degree in law.

While Lord wrote twelve bestselling books on such subjects as Pearl Harbor (Day of Infamy, 1957), the Battle of Midway (Incredible Victory, 1967), the Battle of the Alamo (A Time to Stand, 1961), Arctic exploration (Peary to the Pole , 1963) , pre-World War I America (Good Years: From 1900 to the First World War , 1962), Coastwatchers (Lonely Vigil, 1977) and the civil rights struggle (The Past that would not Die, 1965), he is best known for his best-selling 1955 book A Night to Remember about the sinking of the Titanic. The book was made into a popular 1958 British movie of the same name. In writing A Night to Remember, Lord took the time to track down 63 Titanic survivors to get their stories and wrote a dramatic, minute-by-minute account of the ocean liner's sinking on her maiden voyage. He also authored another book about the Titanic titled The Night Lives On, published in 1986.

Shortly after going to work as a copywriter for the J. Walter Thompson advertising agency in New York City, Lord published The Fremantle Diary, edited and annotated from the journals of the British officer and Confederate sympathizer, Arthur Fremantle, who toured the South for three months in 1863. It was a mild but surprising success in 1954, when Mr. Lord was well into completing A Night to Remember.

In his later years, Lord was renowned for his knowledge of the Titanic catastrophe, frequently lecturing at meetings of the Titanic Historical Society. In 1997, Lord served as a consultant to director James Cameron during the filming of the movie Titanic. The "sequel" to Titanic, Ghosts of the Abyss is dedicated to Lord's memory.

Lord, a lifelong bachelor, died after a long struggle with Parkinson's disease at his Manhattan home at the age of 84. Noted historian David McCullough said of Lord at his death, "He was one of the most generous and kind-hearted men I've ever known, and when I had stars in my eyes and wanted to become a writer, he was a great help. I'll always be indebted to him."

Walter Lord is buried in the Lord family plot at historic Green Mount Cemetery in Baltimore, marked by a marble bench listing the books he authored.

In 2009 Jenny Lawrence edited and published The Way It Was: Walter Lord on His Life and Books. In the late 1980s Lawrence recorded hours of interviews she had with Lord in which he discussed his writing and life. After chapters on his early life in Baltimore and up to his time with the OSS in London and Paris, chapters are devoted to the research and writing of each of his books.

Lord published twelve historical works:

• The Fremantle Diary (1952) (ed.)
• A Night to Remember (1955)
• Day of Infamy (1957)
• The Good Years (1960)
• A Time to Stand (1961)
• Peary and the Pole (1963)
• The Past That Would Not Die (1965)
• Incredible Victory (1967)
• The Dawn's Early Light (1972)
• Lonely Vigil (1977)
• The Miracle of Dunkirk (1982)
• The Night Lives On (1986)