Zenonas was born in Liepaja to a young Lithuanian girl, Adele Grigaitytė, and a German officer, George Boll. Zenonas was still young when his parents divorced. He and his mother settled in the village of Sviliai. When Zenonas was about 10 years old, Adelė regained her maiden name Grigaitytė and Zenonas became Grigaitis. Zenonas's mother Adelė was highly educated, spoke several foreign languages, was interested in medicine and worked as a midwife. When Zenonas was 18 years old, his mother remarried to Jeronimas Žiužnys, who had 3 children from his first marriage. Zenonas got on well with his new family and went to dances with his brothers.

In 1944, the family was approached by an old acquaintance, Chaja Gell, who asked them to help - to save her family and the families of several other relatives. The Žiužniai agreed to help, and one night Jeronimas and Zenon drove to Kaunas (about 60 kilometres from their village) in a wagon, and waited at the ghetto fence until Chaja's cousin Chaja Gell and her two children Rachel and Jacob came through the hole in the ghetto fence. Zenonas and Jeronimas brought the Jews to their house and hid them in a hut where they kept food. A few days later, Chasia's husband, Israel Gell, and several other Jews joined the hiding. In total, 14 people were hiding with the Žiužnys, two of whom were children at the time. Among those in hiding was an architect who invented a place for everyone to live under the kiln. They used to sneak in through the fireplace. Through the iron bars of the kiln - they would take them off and everyone would crawl in. Inside, they made bunks and ventilation, so that you could somehow manage to be. The Žiužnys helped these people for 9 months. Zenonas used to take a horse with a so-called "bricka" and bring a lamb, a piglet or a calf, Jeronimas would go to the forest to hunt, and Adele took care of food, hygiene of the hidden people, and all the other needs.

After the war, the people who were rescued left a written thank-you note to the Žiužnis and Grigaitis, which was signed by all 14 of the rescued people: Israel Gelas, Chazia Geliene, Rachel Gelaite, Jacob Gelas, Codik Bleiman, Boruch Intriligator, Akiva Zaitsev, Abram Rafaika, David Richman, Riva Kaganiene, Jacob Kagan, Genoch Kagan, Itzik Michelson.
On 16 May 1944, Zenonas married Elvyra Nemanytė. The couple had 3 children - Vidutė in 1945, Regimantas in 1950 and Skaistė in 1956.

Zenonas worked as a driver and dispatcher at the collective farm in Josvainiai. He had a good heart and always tried to help others. Zenonas could get up at 4am when asked for a lift, just to help. His neighbours not only loved him but also respected him. When Zenon died, the whole town gathered for his funeral.