Nassiri served as the commander of the Imperial Guards. He was arrested by the followers of Prime Minister Mohammad Mosaddegh when he delivered two decrees of the Shah to the prime minister. A personal friend of the Shah, Nassiri had gained notoriety for removing Mosaddegh from power (see Operation Ajax). He was appointed head of SAVAK following the failure of General Hassan Pakravan, the previous director, to prevent the assassination of Prime Minister Hassan-Ali Mansur on 21 January 1965. Nassiri was also made deputy prime minister. He served in the post until 6 June 1978 when he was dismissed by the Shah. Then Nassiri was appointed ambassador of Iran to Pakistan.
Nassiri was called back from Pakistan and was arrested together with 60 former officials on 7 or 8 November 1978. He was imprisoned with several other high-ranking officials, including Pakravan and former Prime Minister Amir-Abbas Hoveyda. When the Shah left Iran on 16 January 1979, Nassiri remained in prison until the fall of Shapour Bakhtiar's government on 11 February.
On 16 February he was executed by firing squad after a summary trial.