Achievement in Biology in Kenya Certificate of Secondary Education is low despite its significance in the society; this is demonstrated by the low performance scores of the learners in biology. The purpose of the study was to see if there were any differences in learner academic attainment in Biology amongst students instructed using a computer-assisted instructional method and those instructed using outdated methods in secondary schools in Baringo County, Kenya. The study used a Solomon Four -Quasi experimental design. In Baringo County, eight extra-county secondary schools were chosen using purposive sampling. The stratified random sampling approach was employed in the sampling school for the study. A total of 324 biology students were chosen from a stratified sample of extra-county secondary schools. Form three biology students were randomly selected and allocated to the experimental and control groups in selected schools with more than one stream. The investigational category were instructed by aid of a computer-assisted instructional means, whereas the regulator category received outdated instruction approach. As a research tool, the Biology Achievement Test was used. The research device was put through its paces to see how reliable it was. The dependability of the instrument was determined using the split half approach and the correlation coefficient was calculated using the Spearman Brown prophecy formula. The reliability coefficient of the research instrument was 0. 704. Frequencies and percentages were used to evaluate descriptive data, whereas Analysis of Variance and the t-test were used to assess inferential data. Computer aided teaching (CAT) was found to boost learner academic progress in biology when compared to outdated instructional methods. The outcomes of the study will benefit curriculum makers at the Kenya Institute of Curriculum Development in creating a framework and guidelines for integrating, planning, and the enactment of CAT in schools' curriculum, which will help to increase learner academic attainment besides motivation.