Complex Problem Solving (CPS) can be defined as those psychological processes that enable a person to achieve goals under complex conditions, which are characterized by their complexity, connectivity, dynamics, lack of transparency, and polytely. Although many hypothesized influences have previously been tested concerning their relevance for the process of solving complex problems (e.g., general intelligence), results were often found to be rather heterogeneous. As this was found to be partially caused by fundamental differences between measurements of CPS, a new operationalization was used in the present study: Following the Microworld approach, CPS was assessed in the simulation game "Cities: Skylines," as its aptitude as a Microworld could be justified from a theoretical perspective. A parameter for CPS performance was defined to investigate the following hypotheses: Both reasoning and achievement motivation were expected to be positively correlated to CPS performance. Furthermore, a gender difference favoring male participants was expected. Participants in the present study (N = 27) first provided demographic information, then subsequently completed a short test of reasoning and an objective personality test of achievement motivation, and finally were given a mission in a complex scenario implemented in "Cities: Skylines." The results supported all three hypotheses, indicating significant small to moderate positive relationships of both reasoning and achievement motivation with CPS performance, and a significant gender difference favoring male participants in CPS performance. Furthermore, significant gender differences favoring males were also found for reasoning and achievement motivation. Results are discussed and the Microworld operationalization is evaluated.