Fejgin was born in a middle-class Jewish family, and in 1927 he began medical studies, which he never finished. Since 1928, he was a member of the Communist Party of Poland and in 1929 he was sentenced to two years in prison for communist activity. Released, in 1932 Fejgin was incarcerated once again, for four years. After the outbreak of World War Two, Fejgin went to Lwow, where got in touch with Soviet authorities and began working for the Soviets. In May 1943 he joined the Polish 1st Tadeusz Kosciuszko Infantry Division, where he became a political officer. In January 1945, Fejgin took post of director of personal department of the political bureau of the Ludowe Wojsko Polskie.
In October 1949, Fejgin was moved to the Ministry of Public Security of Poland (MBP), where he was appointed director of the Tenth Department, which was protecting the Party from provocateurs. Suspended after deflection of Jozef Swiatlo (December 1953), he was fired from MBP and in 1955 arrested. On November 11, 1957, following thaw in the communist system, he was sentenced to 12 years for breaking law and abusing his powers while interrogating. Among other charges, he was found guilty of torturing 28 persons. Fejgin was released following the 1964 amnesty. In 1985 he became a member of the Society of Fighters for Freedom and Democracy, a state-controlled veterans association and received priviliges of a war veteran. In 1990, however, he was verified and stripped of the privileges due to his Stalinist past. Fejgin appealed this decision to the Supreme Administrative Court of the Republic of Poland, but his claim was rejected. The court emphasized that Fejgin’s post-World War Two actions were harmful to Polish legal system and Polish nation and as such should be condemned.
At the time of his death, Fejgin was still the subject of investigations by National Remembrance Institute.