Immanuel was born on May 13, 1927 in Cairo to Moshe and Simcha Mizrahi. His father came to Egypt from Yemen in 1917, while his mother was from Algeria.
The traditional, Zionist family decided to make aliyah in 1931 with their five children. At first, they settled in Tel Aviv, then moved to Petah Tikva and Rishon LeZion. Eli studied in the Kalischer School until age 16. He was a member of the HaNoar HaOved movement.
In 1945, he joined Lehi and was active in Tel Aviv and Petah Tikva. First, he disseminated propaganda, but after a while he was transferred to the operations division. Eli was involved in blowing up trains, attacking the Kfar Syrkin airfield and robbing Barclays Bank in Tel Aviv, among other operations.
He wore the HaNoar HaOved blue shirt during his time in the underground, as this helped conceal his identity. One time, when he was deputy commander of an operation, he was spotted by his acquaintances from the youth movement, who had also joined Lehi in the meantime. At first, they thought he was a stranger in the field who might endanger the operation, but they were soon surprised to find that he was the deputy commander of it.
In the War of Independence, he participated in the battles of Operation Danny, Operation Horeb, et al. At first, he was in Battalion 89 of the 8th Brigade, but then he was transferred to the Harel Brigade of the Palmach. He was discharged in 1949 and went to work for the Tel Aviv Municipality for a quarter-century.
In 1945, he married fellow Lehi member Rachel Michel. They had a son who became a major in the IDF.