This study investigated the extent to which teachers use computers in teaching and learning in public secondary schools in Arusha city council, Tanzania. Rogers' Diffusion of Innovation theory guided the study. Convergent parallel design under mixed research approach was adopted. Purposive and stratified simple random technique were used to obtain 110 respondents where by 100 were teachers and 10 were heads of schools. Instruments of data collection were questionnaires, structured Interview guide and observation guide. Quantitative data were analyzed descriptively usingmean, standard deviation, frequency tables and percentages, while qualitative data were thematically analyzed. Research ethics were observed whereby sources of information and anonymity of respondents were strictly adhered. The study found that, computers, electrical power and related programs were available in public secondary schools, but technical support as well as supportive and auxiliary infrastructures were found to be averagely available or not available in other schools. Findings gathered that, teachers to a small extent use computers in planning for teaching, lesson preparation, lesson presentation, assessment and evaluation as well as in communication and information sharing. Nevertheless, to a moderate extent teachers use computers for record keeping. Findings exposed that, teachers less often use computers in assessment and evaluation and in record keeping but rarely in planning for teaching, lesson preparation, lesson presentation as well as in communication and information sharing. The study recommended the government to ensure trainings of teachers to enhance their skills in using computer in teaching and learning.