Forester was born in Cairo and educated at Alleyn's School, Dulwich College, and Guy's Hospital, but did not complete his studies there. He married Kathleen Belcher in 1926, had two sons, and divorced in 1945. His eldest son, John, is a noted cycling activist and wrote a biography of his father.

During World War II, Forester moved to the United States where he wrote propaganda to encourage that country to join the Allies. He eventually settled in Berkeley, California. While living in Washington, D.C., he met a young British intelligence officer named Roald Dahl, of whose experiences in the R.A.F. he had heard word, and encouraged him to write about them. In 1947, he secretly married a woman named Dorothy Foster. He suffered extensively from arteriosclerosis later in life.

Forester wrote many other novels, among them The African Queen (1935) and The General (1936); Peninsular War novels in Death to the French (published in the United States as Rifleman Dodd) and The Gun (filmed as The Pride and the Passion in 1957); and seafaring stories that did not involve Hornblower, such as Brown on Resolution (1929); The Captain from Connecticut (1941); The Ship (1943) and Hunting the Bismarck (1959), which was used as the basis of the screenplay for the 1960 film Sink the Bismarck! Several of his works were filmed, most notably the 1951 film The African Queen, directed by John Huston. Forester is also credited as story writer for several movies not based on his published fiction, including Commandos Strike at Dawn (1942).

He wrote several volumes of short stories set during the Second World War. Those in The Nightmare (1954) were based on events in Nazi Germany, ending at the Nuremberg Trials. Stories in The Man in the Yellow Raft (1969) followed the career of the destroyer USS Boon, while many of those in Gold from Crete (1971) followed the destroyer HMS Apache. The last of the stories in the latter book – "If Hitler had invaded England" – offers a plausible sequence of events starting with Hitler's attempt to implement Operation Sea Lion, and culminating in the early military defeat of Nazi Germany in the summer of 1941.

His non-fiction seafaring works include The Age of Fighting Sail (1956), an unusually candid account of the sea battles between Great Britain and the United States in the War of 1812.

In addition to his novels of seafaring life, Forester also published two crime novels, Payment Deferred (1926), and Plain Murder (1930), and two children's books. One, Poo-Poo and the Dragons (1942), was created as a series of stories told to his younger son to encourage him to finish his meals. George had mild food allergies that kept him feeling unwell, and he needed encouragement to eat. The second, The Barbary Pirates (1953), is a children's history of those early 19th-century pirates.