The rapid increase in Tanzanian primary school enrollments in the last decade was prompted by the government to develop the Secondary Education Expansion Policy. My study, therefore, explored the impact of this policy on teachers' professional lives. A qualitative approach was adopted to gain detailed insights into the phenomena under investigation. Data were collected from 30 participant teachers from four community secondary schools in Tanzania through interviews and document analysis. Overall the findings revealed that the government's shortcomings in hiring support staff prompted teachers to perform extra duties along side teaching. It also found that the lack of the government's commitment to rewarding teaching quality exacerbated teachers' engagement in other income-generating activities. Teachers' engagement in these non-teaching tasks both in school and out of school affected their own professional identifies which subsequently impacted on their teaching competence beliefs. These findings recommend that in order to enhance the quality of teaching and learning, the government of Tanzania must improve teachers' welfare by employing enough support staff to assist in teaching and learning.