Mordechai was born in 1929 in Jerusalem to Ita and Yehuda Streisauer, the fourth of six children. His parents made aliyah in 1922, and the family lived in the Rand Houses. The father studied in yeshiva, specializing in Kabbalistic studies and writing compositions which were very well-respected. However, the family’s financial situation was very difficult, and Mordechai had no choice but to halt his studies at Talmud Torah Etz Hayim. His teacher there was Rabbi Aryeh Levin, the prisoners’ rabbi. He engaged in various pursuits, but at age fourteen, he moved to Netanya to work in diamond polishing. A year later, he returned to Jerusalem.
In 1945, he was recruited for Lehi and joined a cell together with five other recruits. They heard lectures and trained with pistols and grenades. Finally, he was assigned to the recruitment department. He also participated in surveillance and putting up posters. He remembers two incidents from that time. One time, he and some other youths ran into a Jew on his way to the synagogue, and he began screaming for the police. Another time, when he hid in the entryway of a house because of an armored vehicle that was pursing him and his friends, one of the residents invited him into his apartment and offered him tea; this was not the typical response Lehi members received at the time.
Mordechai spent most of his time recruiting new members, but meeting them in open fields seemed inadequate to him. He approached the principal of the Geula School, Shalom Friedman, and sought permission to use one of the classrooms after school hours. The principal started screaming about “these brazen dissidents,” pointing to a ring of keys on his desk and shouting, “Why, these dissidents might carry off my keys and copy them!” Mordechai took the hint, and the result was not only that he gained access to a free classroom, but that they had a heated meeting-place for the cold nights of winter, a location for meetings, discussions and admissions boards.
Mordechai also took part in the operation against Brigadier Davis, which was scuttled shortly before it was supposed to take place. He fought in the battles of Der Yassin and Ein Kerem and was part of the attempt to breach the walls of the Old City. After the Bernadotte assassination, he was arrested and detained for three months.
In 1950, he married his girlfriend Beracha, a former IZL member, whose sister and brother-in-law he had recruited for Lehi. They had four children and numerous grandchildren. In 1954, he began working as a division head at Rafa Laboratories, a pharmaceutical company. He worked there for 38 years, a factory manager for 30 of them. Without a formal education, he took courses to complete his studies while he worked. Afterwards, as a pensioner, he volunteered at Shaarei Zedek Hospital in Jerusalem.