Taherzadeh was born into an Iranian family who have had a strong association with the Bahá'í Faith since its inception, in Yazd, Iran. He studied electrical engineering at the University of Tehran, and then moved to Coventry, England in 1948 and pursued advanced studies. He moved to Ireland in 1950 working as the chief engineer of an industrial concern until 1988. He married twice, first to Zarin Moosezadeh Cohen with whom he had two children, a son Tahir Ronald and a daughter Bahiyyeh Vida. He married Belfast-born Lesley Gibson in the 1970s and they had two children, a daughter Maryam and a son Bahhaj. At the time of his death, he had several grandchildren.
Taherzadeh served on the National Spiritual Assembly of the Baha'is of the British Isles from 1960 to 1971. He was elected to the National Spiritual Assembly of the Baha'is of the Republic of Ireland when it was formed in 1972 and was appointed in 1976 to the European Continental Board of Counsellors, a senior advisory body. He was elected to the Universal House of Justice in 1988.
A prolific writer and gifted public speaker, Taherzadeh wrote several books on the Bahá'í history and teachings, which included a four volume study of the life and writings of the founder of the Bahá'í Faith.