Performance in Mathematics among pupils in lower primary schools in Kenya is a problem that continues to be a concern to parents, teachers and stakeholders in education. Teacher related factors and in particular teacher preparedness has been cited as a major contributing factor to poor teaching methods which fundamentally translates to pupils' poor performance. The purpose of the study was to evaluate the influence of teacher preparedness on pupils' performance in Mathematics in lower primary schools in the Aberdares region of Kenya. The objectives of the study were to; evaluate the influence of teachers' preparation of lesson plans on pupils' performance in Mathematics in lower primary schools and assess the influence of teachers' preparation of schemes of work on pupils' performance in Mathematics in lower primary schools from the Aberdares region in Kenya. The following hypothesis were tested; Ho1: There is no statistically significant relationship between teachers' preparation of lesson plans and performance in Mathematics among pupils in lower primary schools, Ho2: There is no statistically significant relationship between teachers' preparation of schemes of work and pupils' performance in Mathematics in lower primary schools. The study adopted the descriptive survey research design. The study was guided by the Social Constructivism Theory (SCT) advanced by Vygotsky (1978). The target population for the study consisted of all the 385 teachers and 1320 pupils in the public primary schools in Aberdares region of Kenya. A sample of schools was selected using Gay's 10-20% sampling principle which yielded a sample size of 77 teachers and 264 pupils. Data for the study was collected using questionnaires administered to the respondents. The t-test statistic was computed to test the hypothesis which stated that there was no statistically significant relationship between teachers' preparation of lessons and pupils' performance in Mathematics. The t-test yielded a p-value = 0.027 which was less than the α-value of 0.05 hence the hypothesis was rejected. It was concluded that there were differences in pupils' performance in Mathematics depending on teacher preparation of lesson. Regarding the preparation of schemes of work, the computed t-test statistic yielded a p-value = .039 which was less than the p-value of .05. Therefore the null hypothesis was accepted. It was concluded that the pupils Mathematics mean scores were relatively the same regardless of whether the teacher prepared schemes of work or not. It is recommended that there is need for teachers to institutionalize as a best practice the preparation of professional documents before commencement of teaching.