Admoni was recruited into the Haganah in 1946, and was assigned to Shai (the Haganah’s information service). He then enlisted in a preparation camp for the Palmach, and continued his service in Shai. Upon the establishment of the State of Israel, he served in the military intelligence corps. He was discharged in 1950, and traveled to the University of California, Berkeley, to study international relations.
After completing his studies, he joined the Mossad in 1954. Admoni served in senior command positions in the Mossad headquarters and abroad. He served as a deputy and then second to Mossad Director Yitzhak Hofi. In 1982, Nahum Admoni was appointed Director of the Mossad by Prime Minister Menachem Begin, on the recommendation of Yitzhak Hofi, and became the first Director to have grown within the Mossad. Admoni served as Mossad Director under three Prime Ministers: Menachem Begin, Yitzhak Shamir, and Shimon Peres.
Admoni began his term as Mossad Director after Operation Peace of the Galilee (the First Lebanon War) began, and extensive areas in Lebanon were under IDF control. During this period, the Mossad continued to exploit its ties and sources in Lebanon to obtain extensive information on the combat arena.
The main part of Operation Moses for bringing Ethiopian Jewry to Israel took place during Admoni’s time. The Mossad’s battle against Iraqi non-conventional weapons was also ongoing. Admoni coordinated this battle back when he was second to the Mossad Director. During his term, the Mossad captured “Nuclear Spy” Mordechai Vanunu, and brought him to trial in Israel.
Nahum Admoni urged to renew intelligence relations with Jordan, and these relations expanded and deepened in many spheres. During his time as Director, the Mossad established covert ties with additional countries that did not have official relations with Israel.
He completed his term in 1989, and was appointed general-manager of Mekorot Water Company. Since retiring, he has served on the board of directors of a number of companies. He currently serves as president of the Mossad’s pensioners association.