Martha Marek (born Martha Löwenstein on 10 October 1897 in Vienna) was orphaned young and raised by a family of modest means. In 1919 she began a relationship with Mortice Fritch, a wealthy 74-year-old department-store owner, who paid for her education at finishing schools in France and England. While still involved with Fritch, she began an affair with engineer Emil Marek. Fritch died in August 1923 and left Martha his estate in the Vienna Woods; she married Emil in 1924, and within two years the couple had spent the inheritance.
In 1925 the Mareks came to public attention for the first time. Emil lost a leg in a wood-chopping accident on 25 June 1925, one day after Martha had taken out an insurance policy on him. The insurer refused to pay, and forensic examination in the resulting lawsuit found that the injury had been self-inflicted. In April 1927 a court found the couple not guilty of insurance fraud but guilty of bribing a public official — a coroner's assistant, who received a six-week sentence — and gave the Mareks a four-month sentence they had already served on remand. The insurer nonetheless settled out of court, though legal fees consumed much of the payout, and Emil's later business ventures as an inventor failed, leaving the family in financial hardship.
Emil Marek, whose health had been poor since the amputation, died unexpectedly on 31 July 1932. The couple's two children then developed similar symptoms, and their seven-year-old daughter died shortly afterward, after Martha had taken out life insurance on the children. Martha's aunt, Susanne Löwenstein, named Martha her sole heir and died in 1934 with symptoms matching those of Emil and the daughter. After that inheritance was spent, Martha took in a tailor, Felicitas Kittenberger, as a subtenant and persuaded her to name Martha as beneficiary on a life-insurance policy; Kittenberger died shortly afterward.
Kittenberger's son grew suspicious of his mother's death and filed a complaint. Separately, Martha was arrested after faking the theft of paintings to claim further insurance money, and the publicity from that case prompted Kittenberger's son to come forward with his suspicions. Investigators exhumed the bodies of Emil Marek, the daughter, Susanne Löwenstein, and Felicitas Kittenberger, and found that all four had been poisoned with Zelio, a thallium-based rat poison that was freely available at the time. A surviving son was also found to have been poisoned after Martha insured him, but he survived.
At trial, Martha Marek feigned seizures and blindness and had to be carried into the courtroom in a specially built chair. A jury convicted her and sentenced her to death on 19 May 1938. Under the tradition of the earlier Austrian republic, a woman's death sentence was expected to be commuted to life imprisonment by presidential pardon, but Austria had by then been annexed to Germany, and Adolf Hitler rejected the clemency petition. Marek was executed by guillotine at the Vienna Regional Court on 6 December 1938, the first person executed there on a guillotine recently brought from Berlin.