Josef Mašín was born at Lošany near Kolín. He was a member of the Czechoslovak Legions fighting in Russia (1916–1921) and later an officer in the Czechoslovak Army (commander of an artillery regiment). After the occupation of Czechoslovakia by Nazi Germany he, together with Josef Balabán and Václav Morávek, formed a resistance group concentrated on intelligence gathering and sabotage.
While more resistance groups existed, this one, aptly named Tři Králové (Three Kings), is the most known among the Czech public. Mašín was captured by the Gestapo on May 13, 1941. After being tortured, he twice attempted suicide. As part of the German retaliatory measures for the assassination of Reinhard Heydrich he was executed in Prague, aged 45. His body was disposed of at Strašnice Crematorium. His wife was imprisoned for several months.
After the war, Josef Mašín received a posthumous promotion to Brigadier General. His sons - then 13 and 15 years old - got Medals for "personal bravery during the war" from president Edvard Beneš.