The aim of this study is to address and analyse pre-service teachers' mathematical reasoning skills in relation to mathematical thinking processes. For these purposes, pre-service teachers' mathematical reasoning skills namely generalising/abstraction/modelling, ratiocination, development and creative thinking skills and the relationships among these skills are examined. Apart from these, it is explored whether grade level and gender have an effect on the application of these skills. The study is based on a mixed method research design and is carried out with 197 pre-service teachers of different grade levels in the school of education of a public university. One of the data collection tools used in the study is mathematical thinking and reasoning skills test which was developed by Basaran (2011) and comprises 21 open-ended questions on real-life problems. The second one is the Mathematical Reasoning Skills and Indicators developed by the researchers in the light of a study by Alkan and Tasdan (2011). Content analysis is performed on the data gathered from the pilot study conducted as the first step of the data analysis and the content of the quantitative data analysis is defined. As the second step, some parametric and nonparametric tests are utilized using the SPSS 15.0 software. As a result of the study, it has been revealed that pre-service teachers' scores on generalising/abstraction/modelling and ratiocination skills are close to average whereas their scores on development and creative thinking skills are below average. It has also been concluded that all the relationships among pre-service teachers' reasoning skills are significant and that correlations among the skills which are associated with stages that follow one another are stronger than the others. Another result of the study is that, in relation to the gender variable, there is a significant difference among the scores concerning generalising/abstraction/modelling and ratiocination skills, yet there are not any significant differences among the development and creative thinking skills scores. In relation to the grade variable, on the other hand, there are not any significant differences among the scores concerning generalising/abstraction/modelling (GAM) and reasoning (R) and improving (I) skills, yet there are significant differences among the creative thinking skills scores of freshman and sophomore pre-service teachers and among those of the sophomore and junior pre-service teachers. Results of the present study are discussed in relation to the relevant literature and some recommendations for future studies pertaining to the subject and to learning environment quality are presented.