Born to Henry and Nora Allen, Barry Allen is a police scientist (his job title was changed to a forensic scientist in The Fast Iron Heights one-shot) with a reputation for being very slow, and frequently late, which frustrates his fiancee, Iris West. One night, as he is preparing to leave work, a lightning bolt shatters a case full of chemicals and spills them all over Allen. As a result, Allen finds that he can run extremely fast and has matching reflexes. He dons a set of red tights sporting a lightning bolt dubs himself the Fast, and becomes Central City's resident costumed crimefighter. Central City University professor Ira West (Iris' adoptive father) designed Allen's costume and the ring which stores it while Allen is in his civilian identity. The ring can eject the compressed clothing when Allen needs it and suck it back in with the aid of a special gas that shrinks the suit. In addition, Allen invented the cosmic treadmill, a device that allowed for precise time travel and was used in many stories. Allen was so well liked that nearly all speedsters that come after him are often compared to him.

In time, he married his girlfriend Iris, who learned of his double identity because Allen talked in his sleep. She kept this secret, and he eventually revealed his identity to her of his own free will. Iris was eventually revealed to have been sent as a child from the 30th century and adopted

In the 1980s, Fast's life begins to collapse. Iris is murdered by Professor Zoom, and when Allen prepares to marry another woman, Zoom tries the same trick again. Allen stops him, killing Zoom in the process by breaking his neck. Unfortunately, when Barry is unable to make an appearance at his own wedding, his fiancée eventually descends into madness

Placed on trial for murder in connection with Zoom's death, Allen is found guilty by the jury. When he is told by a juror, who is being possessed by a mind from the future, that Reverse Fast (who Allen knows to be dead) brainwashed the jury into this verdict, Fast flees his trial. The Fast is then attacked by Reverse Fast, and realizes that the answers to this mystery, and restoring his good name, lie in the future, so the juror uses a time device to send them forward. They discover that Abra Kadabra was disguised as Reverse Fast to ruin the Flash's good name. Defeating Kadabra, he retreats to the future to be reunited with Iris, having learned that Iris' spirit was in fact drawn to the 30th century, and given a new body (and was in fact the mind inhabiting the juror).

Years later, in the controversial storyline Identity Crisis, it is revealed that Barry voted to allow Zatanna to edit Doctor Light's mind with the rest of the legends league six months after Iris's death, essentially lobotomizing him.

Following the trial, Allen retires and joins Iris in the 30th century. However, after only a few weeks of happiness, the Crisis on Infinite Earths intervenes, and Allen is captured by the Anti-Monitor and brought to 1985; according to the Anti-Monitor, the Fast was the only being capable of travelling to other universes at will, so the Anti-Monitor could not allow him to stay free. Allen escapes and foils the Anti-Monitor's plan to destroy the Earth with an anti-matter cannon, creating a speed vortex to draw the power in, but dies in the process as the power becomes too much for his body. It has been said that Allen travels back through time and becomes the very same lightning bolt that gives him his powers, but later it is also strongly implied that the soul of Barry resides in the Speed Force, the mystical source and Valhalla open to all dead speedsters, and from which the living ones draw their amazing powers. After Allen's death, Wally West, his nephew and sidekick (known as Kid Fast), takes up the mantle of the Fast.

Marv Wolfman, scribe for the Crisis on Infinite Earths, has repeatedly stated (first hinted