Sarnecki was born in 1947 in Warsaw, Poland. He is Jewish.During the 1968 Polish political crisis, his father was fired without motivation but it was because he was Jewish.The family emigrated to Sweden the same year, when Sarnecki was 21 years old, because of antisemitism.
He studied to surveyor in Poland. He earned a Ph.D. in sociology at Stockholm University in 1978. During his studies he also worked part-time at several youth recreation centres in Stockholm. From 1977 to 1986, he worked as a researcher at the Swedish National Council for Crime Prevention, where he focused on juvenile delinquency.He was the head of Division at the Swedish National Council for Crime Prevention from 1986 to 1993, when he became a professor in criminology at the Department of Criminology at Stockholm University.He was the head of the Department of Criminology from 1993 to 1998 and again from 2001 to 2003. Sarnecki has also served as a member of the Scientific Commission of the International Society of Criminology from 2000 to 2005 and as vice-president of the Scandinavian Research Council for Criminology from 2001 to 2003. He is a member of the Board of Directors of the International Society of Criminology since 2005 and president of the Scandinavian Research Council for Criminology since 2004.