Herzog was born in Lomza, Poland. At the age of nine he emigrated with his parents and the family settled in Leeds, England, where his father received the position of a rabbi. Yitzhak was educated by his father, who did not want to entrust his son to a center of Talmudic studies and remained his private teacher for as long as it was feasible.

Herzog studied in England and France and traveled throughout Great Britain with Rabbi Jacob David Werner (the "Ridvaz"), one of the greatest Talmudic scholars of his time. After being tested by him for weeks, Herzog was proclaimed to be one of the world's outstanding Talmudists.
At the same time he studied Oriental languages at the Sorbonne in Paris, and classics and mathematics at the University of London, where he received his doctorate. In his dissertation "The Royal Purple and the Biblical Blue", he identified the shellfish that provided the "techelet" or blue-purple dye for the tallit (prayer shawl) in ancient Israel.

In 1916, Yitzhak Herzog received his first appointment as Rabbi of Belfast, in Northern Ireland, where he served until 1919. He became Rabbi in Dublin and in 1921 he received the title of Chief Rabbi of Ireland. The family stayed in Dublin until 1936. He maintained excellent relations with personalities from State and Church and established a lifelong friendship with Eamon de Valera, the Irish Prime Minister.

In 1936, Herzog was invited to become Chief Rabbi of Eretz Yisrael. Chief Rabbi Kook had passed away; the process of selecting a successor was drawn out and politically encumbered. The Chief Rabbi was a central figure in the leadership of the Yishuv, representing the Jewish community to Jews all over the world, as well as to Muslims, Christians and the British Mandatory Authority. He served in this position until 1959.

His son, former Israeli President, Chaim Herzog, wrote in his autobiography "Living History":

My father had won, thirty-seven votes to thirty-three, and we rushed to cable the news to my parents in Ireland. The election marked a major advance on the part of the Orthodox community in Palestine. It was a Statement that they were adapting to changing circumstances. In many ways, my father's appointment was a breakthrough, although sadly the progressiveness he brought with him had not been maintained over the years ... My father was a truth-seeker, and it was important to have such a man as a leader in the religious community. It is still important."

In 1940, the "Va'ad haYeshivot" was founded to supervise advanced Talmud academies. Herzog was its founding President and in charge of the financial support for Yeshivot in the Yishuv. It was also at Herzog's initiative that the Hechal Shlomo building, housing the Chief Rabbinate, was erected in Jerusalem.

In the years before, during and after World War II, Herzog represented the Jews of Palestine and of the whole world. After Hitler's attitude towards the German Jews had become obvious, Herzog tirelessly and repeatedly warned the leadership of the European Jewry about the impending danger. He wrote impassioned letters to the Chief Rabbi of Kovno in Lithuania, urging the community to leave as soon as possible. His concern about European Jewry tpok him to London in 1940, to the USA and South Africa in 1941, 1943 to Turkey and 1944 to Cairo. In 1940 Herzog gained permission from the Soviet Union for teachers and students of Lithuanian and Polish Yeshivot to cross the eastern border of the USSR.

In 1946 he went on a rescue mission to redeem Jewish children from the Catholic churches and monasteries where they had been hidden during the war. He traveled through Europe to restore the children to their rightful homes or to Israel. In many places, these tens of thousands of children were not being allowed to reassume their Jewish identity, or return to the Jewish community. Many children were not even aware that they were Jews. Herzog's hope for the help of Pope Pius XII was not fulfilled, but he was supported by a Jewish committee in Switzerland, Catholic authorities and Queen Wilhelmina of the Netherlands and able to bring thousands of children back to Israel.

Herzog visited Jewish underground fighters who had been interned by the Britons in Kenya.

There was one moving story about my father's trip to see the internees. En route, the plane landed in Jedda, in Saudi Arabia. A high dignitary met my father and took him into the refreshment tent, prepared for the VIPs only. And VIPs did not include Jews. When my brother asked the dignitary why this warm and gracious welcome, he replied, speaking of my father: "I can see he is a man of God."

Herzog was very popular and respected also by the secular community, above all in the kibbutzim. As Chief Rabbi, he was also President of the Supreme Religious Court of Appeals and the Rabbinical Council. Through his publication of rabbinical decrees, he was responsible for family law issues and a significant progress in the reconciliation of modern life with the halachic (Jewish religious) demands. After Independence, he was one of the major halachic authorities who endorsed the reading of Hallel on Yom Haatzmaut.

Other References:
Biography
Techelet http://www.tekhelet.com/pdf/mendel.htm