Cancer is associated with discomfort and many changes in patients’ lives to which they must adapt. The main objective of the study was to assess the use of the mini-MAC questionnaire scale among persons diagnosed with malignant cancer and to develop standards allowing differentiation of patients with diagnosed cancer in terms of their style of adjustment to the disease. The mini-MAC questionnaire is a widely used tool in assessing coping strategies among cancer patients. Sten standards have been developed to determine the level of results on the questionnaire scales in the low–average–high categories. The study included 1187 patients diagnosed with malignant cancer who are covered by outpatient care at the Maria Sklodowska-Curie Institute—Oncology Center in Warsaw. The questionnaire concerning mental adjustment to cancer was used (mini-MAC). Patients with cancer most often adopt strategies of fighting spirit and positive reevaluation. The variables that differentiate the results most significantly include gender, presence of metastasis, and the state of undergoing chemotherapy. The mini-MAC questionnaire should be a tool for psycho-oncological diagnosis of patients’ attitudes towards cancer. The obtained results indicate that cancer patients are characterized by their constructive style of adjustment to the disease.