The modern propensity toward multi-disciplinary research and the alignment of scientific knowledge from different fields has led to significant overlap of the field of Biological sciences with other scientific disciplines. Modern principles of other fields such as Chemistry, Medicine, and Physics are integrated with those of Biology in areas such as Biochemistry, Biomedicine, and Biophysics, making Biology a valuable subject to humankind. However, the government’s inability to retain qualified and experienced teachers in some schools due to high turnover impacts negatively on the students’ learning achievement in this subject. This paper looks into the influence of school working conditions of teachers’ turnover on learning achievement in Biology in public secondary schools in Garissa County. The study was conducted using pragmatic worldview and adopted a convergent mixed-methods design. With a target population of 2786 respondents from 40 public secondary schools, a sample size of 387 respondents was obtained. Using stratified random sampling, 27 schools were selected from which 336 students and 51 teachers of Biology were randomly and purposively drawn respectively. Data collection was done using Biology teacher questionnaire and document analysis guide. The quantitative data was analyzed using descriptive and inferential statistics while qualitative data was analyzed thematically. The study findings showed that school working conditions (r=0.73, p<0.05) had a positive significant correlation with students’ learning achievement in Biology. Qualitative data findings revealed that majority of the teachers intended to transfer from rural to urban schools on the basis of unsuitable working conditions and insecurity. The study concluded that school working conditions of teachers’ turnover influenced low learning achievement in Biology in the study area. The study therefore, recommended strategies that should lower teacher turnover and increase learning achievement in Biology as follows; school principals should provide good leadership and adequate instructional resources for effective teaching of Biology while the government should post security officers to every learning institution in Garissa County to safeguard the non-local teachers’ safety and security