The heart of engineering lies in problem solving. Critical thinking and mathematical thinking are inexorably linked and indispensable in complex engineering problem solving. However, the preparedness of prospective engineers in terms of having the required skills to face future challenges in the engineering workplace is still questionable and worrisome. Therefore, a study to understand the application of these two types of thinking in professional engineering practice is urgently crucial. This study focuses on having insight into the interrelation and interaction among pertinent elements of critical thinking and mathematical thinking used by engineers in professional civil engineering design practice, in the form of a substantive theory. This paper presents qualitative research using a modified grounded-theory method. Semistructured interviews were conducted with eight informants from engineering firms through theoretical sampling. Data were analyzed using modified grounded-theory analysis with analytic tools to interrelate the inductive codes, which contrasts with the more usual grounded-theory analysis. The substantive theory, named math-related critical thinking, explains six essential processes used in professional engineering design for justifying decisions reasonably. The substantive theory provides useful information for prospective engineers and engineering education, along with some implications for designing engineering curricula.