Mrazek was born on October 22, 1925 and lived in the industrial Chomutov region of north-western Bohemia, where he worked as a driver. He was married with children.
Between 1951 and 1957 he attacked women and girls in fields, woods and on paths on the edges of towns, shooting them with a pistol or beating them with a piece of wood. Seven died; there were also rapes and a large number of thefts, and he interfered with several of the bodies after death.
He was arrested on March 17, 1957 after ballistics tied the pistol to the series. He was tried, convicted of seven murders and a long list of other offences, and hanged at Pankrac prison in Prague on December 30, 1957. Nine months from arrest to execution.
He is the best-known Czechoslovak serial case before Olga Hepnarova, and the accessible sourcing is Czech-language material written well after the fact. Reporting of crime of this kind in the 1950s Czechoslovak press was minimal by policy, so the contemporary record is slight and most of what circulates comes from later reconstructions.