Born in Jerusalem, Shai gained a BA in history and political science and an MA in communications from the Hebrew University of Jerusalem, where he also studied in the School of Business Administration. He studied communications at the World Press Institute in Minnesota, and worked as a research fellow in the Shorenstein Center in the JFK School of Government at Harvard University.Nachman holds a Ph.D. in political science and communications from Bar Ilan University and his thesis, on "Public Diplomacy and Low Intensity Conflict", was awarded the 2012 Tshetshik award for national Security research by Tel Aviv University's Institute for National Security Studies.
During his service in the Israel Defense Forces he served in a Nahal unit, worked in Israel Army Radio as its commander and chief editor, and served as the IDF spokesman between 1988 and 1991, reaching the rank of Brigadier-General. During the Gulf War of 1991, when Israel was targeted by Iraqi missiles, his job was to calm Israelis in sealed rooms. He is remembered for instructing concerned citizens to "drink water".
Prior to the 2009 elections he joined the Kadima party, saying "I think there is a time to come down from the stands and enter the playing field. I know the field well and I have been involved in many ways, but I decided this time to become a player". Kadima Chairwoman, Tzipi Livni, presented Shai at Kadima's Knesset faction, saying he was "part of the attractive face Kadima wants to show the electorate" and that he would be able to contribute to Kadima on issues pertaining to the Jewish world, Zionism and the Diaspora. Shai was placed eighteenth on the party list and entered the Knesset as the party won 28 seats.