This study examined senior pre-service English teachers’ (PSTs) state of preparedness to teach and its sources during their field experience in practicum schools. In a longitudinal descriptive design, including school experience and teaching practicum phases, one-on-one, semi-structured interviews were conducted with 30 PSTs. Cross-sectional and retrospective data were collected and analyzed through constant comparison method. In the beginning of the entire process, the PSTs were observed to be either prepared or not prepared to teach. However, as the field experience continued, those who felt somehow prepared to teach emerged. Besides, despite obvious decrease in their number over the field experience, those who felt prepared to teach were more compared to those who were not prepared or somehow prepared to teach. Although the PSTs attached their preparedness to teach to higher teaching-efficacy perceptions and their unpreparedness to teach to untested teaching competencies before experiencing field experience, having been through the process, they were observed to put strong emphasis on their fulfilled professional and developmental needs by their school-based mentors. Despite variations and fluctuations in the sources that the PSTs attached their preparedness to teach to, and among others such as perceptions of higher teaching efficacy, the findings revealed the pivotal role that school-based mentoring played upon the development of PSTs’ preparedness to teach.