Student holistic engagement, determined by the level of participation and intrinsic interest that a student shows in school, is one of the keys that can be used to unlock the academic potentials of students in learning institutions. In Manga Sub-County, therefore, with a percentage pass ranging from 17.03 to 38.66 in KCSE examinations over a period of 5 years, there has been relatively below average performance of secondary school students in national examinations, meaning that there is need to unlock the potentials of the students. Research has been done on a number of factors influencing secondary school students' academic achievement in Kenya, but scanty information is available on the possible influence of student holistic engagement on academic achievement. The purpose of this study was to investigate the relationship between student holistic engagement and academic achievement of secondary school students of Manga Sub County, Nyamira County, Kenya. The objectives of this study were: to establish the relationship between students cognitive engagement and academic achievement among secondary school students, to find out the relationship between students emotional engagement and academic achievement among secondary school students, to establish the relationship between students behavioural engagement and academic achievement among secondary school students, to examine the relationship between students social engagement and academic achievement among secondary school students, and to determine the relationship between students physical engagement and academic achievement among secondary school students in Manga Sub County, Nyamira County, Kenya. The study was hinged on Self Determination and Sociocultural theoretical perspectives. The concurrent triangulation design, an aspect of the mixed methods approach, was employed. From the target population of 1750 form four students, 35 Principals and 35 Guidance and Counselling teachers, 316 students, 11 Principals and 11 Guidance and counselling teachers were randomly sampled for the study. Questionnaires were used to collect data from the students, while interview schedules were the instruments used to collect data from Principals Guidance and Counselling teachers and student leaders. The validity of the research instruments was determined by experts from the department of Psychology and Educational Foundations of Jaramogi Oginga Odinga University of Science and Technology. Reliability was ascertained by the internal consistency method using Cronbach's alpha, and a reliability coefficient of r above 0.7 was obtained for the questionnaire items. Inferential statistics from quantitative data were analyzed using Pearson's Product correlation and regression analysis with the aid of the statistical package for social sciences (SPSS) version 22. Qualitative data from interviews were analyzed using thematic analysis. The study revealed that: cognitive engagement was a significant predictor of academic achievement among secondary school students studied (r=.376, N=312, p =.OJ); there was a statistically significant but moderate positive correlation (r=.354, N=312, p<.05) between emotional engagement and academic achievement among the students; behavioural engagement was a significant predictor of academic achievement, (r = .327, n=312, p<.05); social engagement was a significant predicator of academic achievement (r=.226, n=3 I 2, p<.05); there was a statistically significant, but fairly weak, positive correlation between student physical engagement and the student academic achievement, (r=.337, n=312; p<.05). The study recommended that school based teacher counsellors should adopt appropriate therapy techniques to enhance emotional engagement and specifically utilize cognitive behavioural therapy techniques during counselling sessions with students in school. To the teacher counsellors, the study may provide eye openers to the various parameters of school engagement that need to be focused on in the course of guidance on school success, as well as counselling on coping strategies with academic rigours.