Alan Turing was born on June 23, 1912, in Paddington, London, to upper-middle-class British parents, Julius Mathison and Ethel Sara Turing. An intelligent child, Turing spent much of his early life fostered in various English homes with his elder brother John—Julius and Ethel lived in India while Julius worked in the Indian Civil Service—and was often a lonely child. As biographer Andrew Hodges put it, Turing’s life was one of “an isolated and autonomous mind.”

Turing was fascinated with science, but he found little encouragement to pursue his interests from his foster homes or even his mother, who was fearful he would not be accepted into English public school. At 13 years old, however, he was accepted into a boarding school called Sherborne School, where he studied advanced scientific concepts like relativity on his own.

At Sherborne School, Turing formed a strong bond with fellow student Christopher Morcom, who inspired him to communicate more and focus on academic success. But Christopher died suddenly of tuberculosis in 1930, devastating Turing, who questioned whether his friend’s mind somehow lived on in matter. He turned to studying quantum mechanics for answers, his emotional pain turning into a scientific and intellectual fascination with the mind and brain that would underlie his later work.

In 1931, Turing began attending King’s College (University of Cambridge), a progressive new home that both fostered his scientific curiosities and helped further define his homosexual identity. Upon graduation, he was elected a Fellow at the college in 1935 and a year later delivered his foundational paper on the universal Turing machine.

This hypothetical singular machine could theoretically compute anything computable or solve any well-defined task when given a set of pre-defined rules or instructions. Impossible to build, his proposed device laid the groundwork for modern computers, earning Turing the posthumous title, “the father of modern computer science.”

Turing left Britain to study cryptography and earn a Ph.D. in mathematics from Princeton University in 1938. He returned to Cambridge to work with the British code-breaking organization, the Government Code and Cypher School, at Bletchley Park (the British government’s wartime communications hub) in September 1939.

During World War II, Turing devoted his brilliance to code breaking. Somewhat resembling a large typewriter, the German cyphering machine Enigma replaced a text’s letters with random ones selected using a set of internal rotors. It could generate billions of possible combinations, making the German military’s coded messages seemingly impossible to understand.

Turing, joined by other mathematicians at Bletchley Park, cracked the Enigma code quickly after he came to the organization. He and his codebreaking colleagues (Cambridge mathematician W. G. Welchman, especially) developed another machine called the bombe, which mimicked the workings of Enigma’s rotors to test potential ciphers.

Bletchley Park installed a prototype bombe called Victory in the spring of 1940. Within a few years, the bombes were cracking roughly two Enigma messages each minute. The German military would later switch to a more complex device called Lorenz, but Turing developed a technique to crack even these messages. Turing's contributions to the war effort saved millions of lives and shorten the war by two to four years, according to some historians.

Once solitary in his life, Turing became the chief scientific figure at Bletchley Park and had garnered attention and respect from his colleagues, who called him “Prof.” He was known to be eccentric and awkward, with characteristics ranging from solitary and gloomy to eager and vivacious.

He found a strong companionship with fellow mathematician Joan Clarke, to whom he even proposed marriage. Though Joan gladly accepted, Turing quickly retracted, telling her of his homosexuality.

Following World War II and still working in secret with the government, Turing moved to London to work for the National Physics Laboratory (NPL), where he sought to “build a brain,” a device akin to modern computers that could store programs in its memory. He spearheaded the design for the Automatic Computing Engine—which NPL deemed too complex and expensive to build at the time—that would influence the design of computers to come.

Turing left NPL in 1948 to work on another computer for Manchester University called the Manchester Mark I. While at Manchester in 1950, he published a famous paper in which he proposed an experiment called the “Turing test” to determine whether a computer could imitate human conservation. This test would become a foundational part of the field of artificial intelligence.

He then turned his attention to trying to understand how patterns in nature, such as a zebra’s stripes, arise, publishing a seminal paper on the topic of morphogenesis.

Turing's scientific work came to an abrupt end in 1952. Police responded to a burglary at Turing’s home and eventually learned of his sexual relationship with a young man named Arnold Murray, who told Turing he knew the identity of the burglar.

Homosexuality being illegal at the time, Turing and Murray were both charged with “gross indecency,” to which Turing pleaded guilty. Rather than lose his Manchester job by serving jail time, Turing agreed to undergo chemical castration—taking estrogen injections to curb his libido, which eventually rendered him impotent.

However, Turing still lost his security clearances and could no longer continue his code-breaking work with the government. Now a security risk, he was also harassed by police surveillance.

Turing was found dead in his bed, with a partially eaten apple next to him, on June 8, 1954, by his cleaner. He died by cyanide poisoning the day before and the coroner deemed his death suicide, the cyanide believed to have been injected into the apple.

However, the apple was never tested—a 2012 BBC article argues that the case for suicide is unsupported and Turing may have accidentally inhaled cyanide during a chemistry experiment, as Turing’s mother claimed.

In 2013, Alan Turing received a posthumous royal pardon from Queen Elizabeth II for his gross indecency conviction. A few years later, the British government announced “Turing’s Law,” a posthumous pardon of thousands more who suffered the same conviction.

In 2014, Actor Benedict Cumberbatch played Alan Turing in an historical drama film of the mathematician’s life, The Imitation Game. In 2021, Turing became the face of the new 50-pound bank note, which went into circulation on his birthday.

Alan Turing — A Short Biography, by Andrew Hodges.
Alan Turing: Creator of modern computing. BBC Teach.
Alan Turing. Stanford Encyclopedia of Philosophy.
Alan Turing, by Jacob Aron, New Scientist. 
Remembering Alan Turing: from codebreaking to AI, Turing made the world what it is today, by Liat Clark and Ian Steadman, Wired.
New AI may pass the famed Turing Test. This is the man who created it, by Erin Blakemore, National Geographic.
Alan Turing: The codebreaker who saved 'millions of lives,' by Prof Jack Copeland, BBC.
Alan Turing: The experiment that shaped artificial intelligence, by Prof Noel Sharkey, BBC.
Overlooked No More: Alan Turing, Condemned Code Breaker and Computer Visionary, by Alan Cowell, New York Times.
What was Alan Turing really like? by Vincent Dowd, BBC.
Alan Turing: Inquest's suicide verdict 'not supportable,' by Roland Pease, BBC.
Royal pardon for codebreaker Alan Turing. BBC.
New U.K. Currency Honors Alan Turing, Pioneering Computer Scientist And Code-Breaker," by Rachel Treisman, NPR.