Klemensas was born in Vilnius, Lithuania. He finished his undergraduate degree in synthetic organic chemistry at the University of Nottingham before starting at the Synthesis for Biology and Medicine CDT in Oxford in 2017. Klemensas has since then moved on to a more interdisciplinary field of chemical biology. He is currently working on developing chemical tools for DNA-modifying enzymes, with a particular interest in the ten eleven translocation (TET) enzyme family. Outside of the lab, Klemensas dabbles in street food gastronomy and powerlifting.