Arthur Prior’s logic was influenced, among others, by logicians from the Lvov-Warsaw school. This paper introduces the impact Leśniewski’s Ontology had on Prior’s logical system. The paper describes the main characteristics of Leśniewski’s Ontology, Prior’s logical system and the manner in which Prior became acquainted with Leśniewski’s logical system. Since Leśniewski was no longer alive when Prior began to develop his logical system and Leśniewski’s papers were not easily available to Prior, this paper also includes Prior’s interpretation of Leśniewski’s logical system which did not always correspond to Leśniewski’s original ideas.