Yaacov was born on July 9, 1917 in the town of Osditich in the Volyn region of what was then Poland, but is now Ukraine. His mother was Sarah Ita and his father was Yosef Finkelstein. In 1925, his parents moved with him and his three sisters to the town of Torczyn. Beginning at age twelve, he was a member of Beitar, and he joined the nationalist cells of IZL beginning in 1938.
In 1939, he participated in an IZL course in Zofiówka, led by Yaacov Polani, sent from the Israel, and his lieutenant, Yaacov Tunkel (Banai). When World War II erupted later that year, they moved into areas controlled by the Soviets.
Yaacov, with some of his friends, set out for the Land of Israel. They moved to Vilna, and from there, via Moscow and Odessa, they made their way to Turkey. However, the Turkish authorities would not let them disembark, leaving them in dire straits until an influential Jew by the name of Shimon Brod intervened. His efforts were successful, and they were allowed to continue their journey. When they reached Mersin, Avresha Blass informed them about the split in IZL. They continued to Tripoli by rail, Beirut by bus and Haifa by taxicab. There they spent a week in the aliyah center.
For his livelihood, Yaacov took on many odd jobs, and in 1942 he began to study diamond polishing. At that time, news of the horrors in Europe began to trickle back to him, and he found out that his entire family had been murdered by Ukrainian rioters, along with the other Jews of Torczyn and the surrounding area.
In early 1943, Yaacov became a member of Lehi, joining the Intelligence Department (Dept. 6). He lived in the Shabazi neighborhood of Tel Aviv, but at times he was compelled to live elsewhere.
The British CID arrived at the factory he worked in to arrest him in 1945, but he was warned and managed to get away. He had to change his name, first to Sapir, then to Scheffler, his mother’s maiden name. He recruited supporters and looked for locations to stow weapons. Later on, he joined the Technical Department and worked on experiments to create explosive materials. For a short time, he was replaced and returned to his former career of diamond polishing, but he soon returned to the production of explosive materials. This lasted until he enlisted in the IDF with his other underground comrades, to the 82nd Battalion of the 8th Brigade, where he served until the end of the War of Independence.
In 1955, he married Rivka (Regina) Glicklich, and they had two sons, Shuki and Tzvika, as well as many grandchildren. After the war was over, he returned to his career in diamond polishing, working in Tel Aviv and Ramat Gan.
As a pensioner, he audited courses at Bar-Ilan University for many years, studying Judaism, history, the Bible and more.