Teachers spend lengthy periods of their lifetime in school work environments and their satisfaction in the job has crucially far-reaching implications. Besides making them feel good about themselves, job satisfaction draws on teachers’ well-being as satisfied teachers are less susceptible to leaving their positions. The aim of this paper was to evaluate teachers’ job satisfaction as a teachers’ turnover factor in influencing learning achievement in Biology in public secondary schools in Garissa County. Guided by Becker’s Human Capital Theory of Occupational Choice, the study was driven by a philosophical paradigm borrowed from the Pragmatic Worldview and adopted a convergent mixed-methods research design. The study targeted 2786 respondents comprising 2691 form three students and 95 Biology teachers from 40 public secondary schools from which 27 schools were picked through stratified random sampling. The sample size constituted 336 form three students and 51 Biology teachers obtained through simple random and purposive sampling respectively. Questionnaire and document analysis guide were used in data collection. Both descriptive and inferential statistics using Pearson Product Moment Correlation and Linear Regression were done. The study established that job satisfaction (r=0.67, p<0.05) had a positive significant correlation with learning achievement in Biology. The study therefore concluded that, teachers’ job satisfaction influenced learning achievement in Biology in public secondary schools in Garissa County. The study recommended strategies that should lower teacher turnover and increase learning achievement in Biology thus; re-visit salary schemes for teachers to ensure they re-visit salary schemes for teachers in public secondary schools to ensure teachers are well paid besides are well paid, teacher promotions should be streamlined and staff development opportunities enhanced.