He cooperated with Princeton University, European Southern Observatory, and for twenty years worked at the Warsaw University Observatory. He has lectured at research institutions in Japan, Germany, France, Belgium, Italy, and China, among other places.

Pietrzyński authored or co-authored more than 400 scientific publications, which appeared in the most prestigious journals, such as Nature, Science, Astrophysical Journal, or Astronomical Journal, accruing more than 23,000 citations.

Moreover, Pietrzyński won many prestigious Polish and international research grants, including the European Research Council grant (ERC), the Nature magazine award for outstanding scientific contribution, Maria Skłodowska-Curie Award in physics from the Polish Academy of Sciences, and the Rector of the University of Warsaw Award. In 2011, together with Dr. Igor Soszyński, Pietrzyński received the Marian Mięsowicz Prize for outstanding achievements in physics from the Council of the Polish Academy of Arts and Sciences. He is a laureate of FOCUS and TEAM programmes of the Foundation for Polish Science.