The purpose of this study was to establish the causes of changes from the traditional method of circumcision to the modern male initiation rite of circumcision among the Ameru of Imenti South Sub-County, Kenya. The period considered was between 1950 and 2016. The topic has been difficult to penetrate because of the secrecy of the traditional circumcision rite. The study used Modernisation Theory (MT) as a tool for analysis. Methodologically, the study used a descriptive research design. The study was carried out among the Ameru of Imenti South Sub-County, because of the modern changes in male initiation rite of circumcision. ata was collected from primary, archival and secondary sources. Interview schedules focus group discussions and observation. The target population was 61,091 adult men from Imenti South Sub-County, and the sample size comprised 50 randomly selected respondents. The main research findings were that changes in the male initiation rite of circumcision were caused by urbanisation, formal schooling, the establishment of health facilities and colonisation, all of which were intended to Christianise the operations. This research contributes to the wider comprehension of the male initiation rite of circumcision among the Ameru of Imenti South Sub-County between 1950 and 2016. The study contributes to the Ameru historiography.