The knowledge and ability of the individuals in a managerial position are to fulfill some specific management activities or tasks. Managerial skills are important for many reasons. Being a manager in an educational setting is a position to act as an effective leader and problem-solver in many simple and complex situations. However, in Thailand, it was observed that some managerial skills need to improve their skills. School managers should strengthen and develop to manage effectively and efficiently. This mixed research surveyed the managerial skills of the school administrators in terms of conceptual skills, human skills, and technical skills in selected private catholic secondary schools in Bangkok, Thailand. The respondents self-assessed their managerial skills in the form of a checklist, and to validate the results, interviews were conducted to determine the professional development needs according to the lowest items assessed by the school administrators. A questionnaire anchored on Robert Katz's theory of managerial skills and developed by Goodarzi (2002) was used to gather data. With regards to the assessment of managerial skills' practice of the school administrators, most of them assessed themselves as very good in terms of conceptual skills, human skills, and technical skills. The professional development needs of the private catholic school administrators in terms of conceptual skills are the identification of informal organization, articulation of strategy, and ability of prediction. On the other hand, school administrators need training in controlling conflict, report generation, and official correspondence in terms of human skills. Finally, they need professional development in financial management, computer skill, and marketing in terms of technical skills.