Harry Harrison was born Henry Maxwell Dempsey on 12th March 1925 in Stamford, Connecticut. His father, Henry Leo Dempsey, changed the family name to Harrison when Harry was an infant, and Harry would subsequently legally change his own name to Harry Max Harrison when he was thirty. The Harrison family moved to Brooklyn when Harry was two, and he grew up in New York, though the family repeatedly moved throughout his childhood as Harry's father, unable to sustain employment during the Great Depression, frequently relocated the family at short notice to avoid creditors. The constant moving made it hard for Harry to make friends, resulting in him instead finding enjoyment in reading pulp magazines, from which he gained his life long love of science fiction. He discovered SF fandom when he was thirteen, and in November 1938, at the tender age of fourteen, he became one of the founding members of the Queens chapter of the Science Fiction League.
Turning eighteen and graduating high school in 1943, and aware he would soon be drafted, Harry attended New Jersey's Eastern Aircraft Instrument School to become a certified aircraft instrument mechanic, thus ensuring he would be drafted into the U.S. Army Air Corps as ground crew. While in the forces he learned of the artificially derived language Esperanto, and, finding that studying it helped stave off boredom, he swiftly became proficient; he would become a lifelong proponent of Esperanto.
After being discharged at the end of the war, Harry attended New York's Cartoonist and Illustrator's School, studying under Tarzan artist Burne Hogarth and alongside Wally Wood. He and Wood became collaborators and shared a studio, selling their services to numerous magazines. Via his illustrations for SF magazines Harry gained membership of the Hydra Club, a group which included many notable SF professional writers such as Lester del Ray, Theodore Sturgeon and Isaac Asimov, and this in turn led to commissions for the magazine Worlds Beyond. When a bout of the flu left his hands temporarily too shaky to draw properly, he tried writing instead, and managed to sell his first story, Rock Driver, which was published in Worlds Beyond's February 1951 issue. Around this time Harry also met his future wife, Joan Merkler, and they married three years later.
After Wally Wood and Harry stopped working together, Harry's comics work gradually shifted more and more away from just being an artist. As well as writing, he became an editor, working on Beware, Girl's Love Stories and Nuts. The backlash against comics triggered by Frederick Wertham saw a contraction in the industry, prompting Harry to shift into editing pulp magazines such as Fantasy Fiction, Sea Adventures and Science Fiction Adventures, while also writing for the Saint comic strip, a role he maintained for five years. Increasingly also writing for these magazines, in 1956 Harry finally took the leap and quit his editing jobs to become a freelance writer, relocating his family to Mexico, where they could live more cheaply on his savings until the writing started to pay for itself.
In 1957 they relocated to England, for much the same financial reasons, and while there Harry wrote a short story introducing what would become his most famous character, James Bolivar DiGriz, a.k.a. Slippery Jim, a.k.a. the Stainless Steel Rat, who would go on to star in several novels.
Around this same time Harry wrote several scripts for Sidney Jordan's Jeff Hawke series, as well as various Science Fiction strips for Amalgamated Press, such as Rick Random; Harrison is frequently credited with creating the character, but since Random debuted in 1954 and Harry's first confirmed credit scripting the space detective wasn't until 1957, it seems this particular credit is misattributed.
In 1958 the Harrisons moved to Capri in Italy to get away from the British weather. Via friends in the SF fandom community, Harry approached Flash Gordon artist Dan Barry when he heard the ongoing newspaper strip needed a new writer, and so became the strip's new scribe for the best part of the next decade. Late in 1959, with his daughter's birth imminent, Harry moved the family back to New York, shortly before the publication of his first novel, Deathworld, which was initially released in installments in the magazine Astounding, starting in January 1960. Harry once again relocated his family, this time to Denmark, where they stayed for the next seven years, and where Harry made the leap from pulp novels (written with the intention of being published in installments in magazines before being gathered together for novel publication) to more serious science fiction. In 1964 he wrote Bill the Galactic Hero, a scathing comedic look at life in the interstellar military, a novel shaped in no little way by his less-than-fond memories of his own military experience. In 1966 he published Make Room, Make Room!, a novel looking at the impact of unrestrained population growth, which would later be adapted into the movie Soylent Green in 1973.
In 1967 the Harrisons moved back very briefly to London, and then to the USA, setting up home in San Diego. While there Harry taught courses in science fiction at Montgomery High School and San Diego University, but in 1974, when San Diego city elected to build a road literally right through their home, the Harrisons returned to London, and then in 1975 to the Republic of Ireland, where they stayed for the next two decades. In the 1980s, while still in Ireland, he authorised 2000 AD to adapt three of his Stainless Steel Rat novels.
In his later years Harry maintained homes in Ireland and Brighton, England; he passed away in the latter on August 15th, 2012, aged 87.