Rejzner was born in 1985 in Kraków, the daughter of two architects. She earned a master's degree in physics in 2009 from Jagiellonian University, and completed her Ph.D. in 2011 at the University of Hamburg under the supervision of Klaus Fredenhagen, with a dissertation on the Batalin–Vilkovisky formalism. After postdoctoral studies at the University of Rome Tor Vergata she joined the University of York in 2013, and was promoted to senior lecturer there in 2017. In 2016 and 2017, she visited the Perimeter Institute as an Emmy Noether Visiting Fellow.