Pro-environmental behavior is a key to environmental sustainability. It is important to know which variables influence the development of pro-environmental behavior and how important direct interaction with nature is for future environmental engagement. The aim of the research was to examine the correlation between childhood interaction with nature and pro-environmental behavior as an adult and the mediators influencing this correlation. The respondents (N = 370) were selected by the snowball method and completed an on-line questionnaire. Bivariate correlation and parallel mediation analysis were done. Previous research studies focused mainly on one type of influence, and this research brought a comparison of three mediators. The results confirmed that interaction with nature in childhood affects future pro-environmental behavior. The main mediator seems to be the affective mediator. The cognitive mediator and interaction with nature in adulthood had only a small mediation effect. Children’s emotional bonding with nature is the important element for the development of pro-environmental behavior. This appears to be a more effective way of ensuring stronger pro-environmental behavior in the future than building environmental knowledge or relying on sufficient interaction with nature in adulthood.