Background: Professional competencies have become a key aspect of Human Resources Management in the new Public Administration paradigm. One of the basic elements of the new Public Administration in Spain is professional skills, understood as the ability of a worker to perform the tasks inherent to a specific job. This paper examines the capacity of the variables Personality, Engagement, Job Characteristics, Job Demands, and Effort–Reward Imbalance to predict Professional Competencies (Professional Responsibility, Professional Risks, and Professional Growth/Development). Method: A multi-occupational sample of 502 employees from the General Body of the Public Administration of Spain (men: 35.1%; women: 64.9%) was obtained through non-probabilistic sampling, and the data collected were processed with the SPSS 26.0 program. Pearson's correlations were calculated to generate the correlation matrix between the predictor variables and the criterion variables. Subsequently, a multiple regression model was applied to test the effects of the predictor variables on the criterion variables for professional skills. Results: Significant correlations were found with Professional Responsibility, Professional Threats and Risks, and Professional Growth and Development, along with several contrast variables and external correlates. The results confirm a significant positive predictive relationship between Professional Competencies and various predictor variables. Conclusions: The findings of this study provide valuable insights for research on the factors that promote professional competencies in Public Administration. The variable Dedication is the best predictor of the Professional Responsibility and Professional Growth and Development variables, while the variable Control is the best predictor of Professional Threats and Risks. These results have direct implications for Human Resources Management in Public Administration.