Marek was born Martha Lowenstein in 1904 in Vienna and was adopted out of poverty by a wealthy department store owner, who left her money. She married Emil Marek, an engineering student, and the two ran through what she had.
Their first fraud, in 1925, was an insurance claim: Emil's leg was severed below the knee, they said by an axe while he was felling a tree. The insurer refused to pay, arguing the cuts showed three separate strokes and could not have been accidental, and both were prosecuted for attempted fraud. The case ran through the Vienna courts for years and made them notorious.
Between 1932 and 1937 she poisoned four people with thallium: Emil; their daughter Ingeborg, an infant; Suzanne Lowenstein, an elderly relative she was nursing; and a lodger, Felicitas Kittenberger. She inherited or claimed on each. The deaths were recorded as tuberculosis and other illnesses.
She was arrested in 1937 after selling a painting that belonged to a victim's estate and after relatives complained. Thallium was found in the exhumed bodies.
She was tried in Vienna, convicted, and beheaded on December 6, 1938, by Johann Reichhart, the Bavarian executioner. Austria had been annexed by Germany that March, and the guillotine and the executioner were both consequences of that. She is one of very few women executed in Austria in the twentieth century, and the annexation is why the method was German rather than Austrian.