He was born in Rychnov nad Kněžnou into a family of a Jewish trader. He started to attend secondary school (Czech gymnasium) there, but due to his bad results he transferred to a secondary school in Prague, from which he graduated in 1912. He then attended the faculty of law at Charles University.
He was employed as a clerk for a short time. During the First World War he was sent on a Serbian and Galician front. After the war he was employed in the Czechoslovak committee on import and export. But he lost his job after he ridiculed the office in one of his short-stories called Kolotoč (The Carousel). The story is about a family that inherits a carousel but due to a hyperbureaucratic committee on import and export they are not able to sell it abroad.
Josef Čapek offered him a cooperation in 1920. Poláček contributed to a humoristic magazine Nebojsa (English Dreadnought). He started writing short-stories, feature-stories and columns using a pseudonym Kočkodan (English Guenon). Shortly after that in 1922 he was introduced to the editor's office of Lidové noviny (a famous newspaper at that time) by the Čapek brothers. The newspaper published his feature-stories and very popular "soudničky" (stories - usually funny - from the court). His work was published in this newspaper until the Nazi occupation came and forbade it with racist laws.
His novels represent one of the most authentic values of the Czech interwar prose. He was close with his humanistic credo to his generation fellows such as Karel Čapek and František Langer. At the same time he reflects in his "humoristic" (but only at the first sight) novel the deep tragedy of the petty bourgeois, small-town and suburban world in which hypocrisy, mental smallness, narrow-mindness and spiritual poverty wins.
Poláček was able to describe different human types - not only in their type variety but also in the art of getting under the mask of their language. At the beginning of his work stand humoristic sketches mostly from small-town environment with caricatured human figures especially from middle-class, often Jewish society.
His first novel was Dům na předměstí (1928) (English A House in the Suburbs) in which he portrayed a rebirth of a "small man" into a dehumanised creature as soon as he is possessed with proprietary instincts to possess. He was widely popular for his humoristic prose such as Muži v offsidu (1931) (Men in Offside), which was made into a movie that year by director Svatopluk Innemann, starring Hugo Haas in the role of Mr. Načeradec, or Michelup a motocykl (1935) (Michelup and the Motorcycle).
The work of his life became a cycle in which he portrayed a small piedmont town during years before the 1st world war. The story is concentrated around the fate of a tradesman Štědrý and his sons. It was supposed to be a pentalogy - 5th part is said to have been written but its destiny is unknown - the books were published in this order: Okresní město (1936) (County Town), Hrdinové táhnou do boje (1936), Podzemní město (1937) (Underground Town) a Vyprodáno (1939) (Sold Out).
During the Nazi occupation in 1941 Poláček's humoristic novel Hostinec u kamenného stolu was published under the name of painter V. Rada. It was made into a movie in 1949. After the Second World War a novel about his childhood in Rychnov nad Kněžnou Bylo nás pět (1949) (There Were Five Of Us) was published.
- Several works by Karel Poláček available on the website of the Municipal Library in Prague(in Czech)