From 1966 he worked as a scientist at Lawrence Berkeley National Laboratory (LBL) and remained on the staff there until 2001, when he retired with emeritus status.

He is famous as one of the co-discoverers of what nuclear physicists call collective flow; this phenomenon consists of fluidic motion exhibited by nuclear matter, such as quarks and gluons, when compressed to a physical state of high temperature and high energy-density.

Among the many highlights of Poskanzer's career at Berkeley Lab, in addition to his co-discovery of collective flow of nuclear matter, he was the first scientific director of the Bevalac accelerator, the co-founder of the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC), and one of the leading organizers behind SPS heavy ion program at CERN.

He was also a co-discoverer of elliptic flow at RHIC, which has proved to be major experimental evidence for the existence of the quark-gluon plasma, an ephemeral state of matter believed to have existed in the first few microseconds after the universe was born.

[6]For the academic year 1970/71 he was a Guggenheim Fellow[7] at the University of Paris-Sud in Orsay.