Monroe Fein was born in Chicago to a middle-class Jewish family in 1923. His father was a lawyer and a donor to Zionist causes. As a child, he excelled in sports such as swimming, running, and boxing.
When Fein entered university, he made good use his physical abilities in encounters with antisemites. Following the outbreak of World War II, Fein cut short his studies and enlisted in the United States Navy. He reached the rank of Lieutenant and was appointed commander of a transport ship His ship saw action in the Pacific, participating in the Marshall Islands campaign. He later served on an aircraft carrier that participated in raids on Japan.
Following the end of the war, Fein was discharged from the US Navy in 1946. He returned to Chicago and found work as the office manager of a publishing house in Chicago.
In Chicago, Fein quit his job and volunteered to help the Zionist cause. He met Irgun representative Avraham Stavsky, and was appointed captain of the Altalena on behalf of the Irgun from its departure point in Port-de-Bouc, France to the newly formed state of Israel in June 1948. Upon arriving in Israel the ship was shelled causing incredible controversy and antagonism between Israeli military and paramilitary groups. After the shelling of the Altalena many members of the Irgun were arrested, including Captain Fein. The shelling, controversy and political fallout that came from the fight are collectively known as the Altalena Affair.
Following the affair, Fein was arrested, but released a week later due to asthma. While recovering, he stayed in the Geula Hotel in Tel Aviv, where he was looked after by an Irgun nurse, Malka Yefet. He then left Israel and returned to the United States.
Following Fein's return to the United States, he studied engineering in Berkeley. His former nurse Malka Yefet eventually also moved to the US, and reestablished contact with him. They met in New York City, developed a relationship, and married on April 21, 1949. In 1952, the couple returned to Israel with their first child, who was about a year old. He did not find a job, and after six months in Israel, they returned to the US after Fein had received a job offer from General Electric. He later worked for a large water and electricity company in California. Malka found work as a nurse in Santa Monica. The couple had four children: Dan, Kenny, Richard, and Sharona.
Fein died in Santa Monica on July 2, 1982, at the age of 59.