Achievement in Mathematics has remained a challenge to many students at secondary school level in Kenya, for example students in Vihiga Sub County. Despite the government’s effort in strengthening the subject, its performance is still wanting. The main objective of the study was to determine the influence of the level of Internal Locus of Control on Mathematics achievement among students in secondary schools in Vihiga Sub-county. The Self-determination theory informed the study. The study adopted Mixed Methods approach and the Sequential Explanatory Design was used. The study targeted 1483 form four students, 35 Mathematics teachers, and 27 teacher counsellors. A sample size of 445 students, 11 Mathematics teachers, and 9 teacher counsellors were selected using stratified random, purposive and purposive sampling techniques respectively. Quantitative data was collected using a Students’ Questionnaire while qualitative data was collected using interviews from students, teacher counsellors and Mathematics teachers. Students’ achievement was assessed using K.C.S.E Exam results of the year 2017. Reliability of the questionnaire was ensured by Cronbach’s alpha and a coefficient of alpha >0.7 was reported. Normality of data was tested by using Kolmogorov-Smirnov and Shapiro-Wilk (W) tests. Trustworthiness of qualitative data was also ensured. Descriptive statistics such as frequencies and percentages were used to analyze quantitative data from questionnaires, while inferential statistics such as Regression Analysis and Pearson correlation coefficient were used to analyze quantitative data. On the other hand, thematic framework was used to analyze Qualitative data. The study found that there was statistically significant, though weak, positive correlation (r=.182, n=396, p<.05) between internal locus of control and Mathematics achievement. The findings showed that internal locus of control predicted the achievement in Mathematics among secondary school students. The study recommended that Mathematics teachers should come up with a variety of learning strategies that would encourage students to develop internal locus of control hence, make them to start believing that they have more control over their Mathematics achievement and that teachers are guides. This would enable students to take responsibility of their learning and achieve highly in Mathematics.