This work studied the effect of teacher mentoring on the professional growth of newly qualified and employed teachers (NQETs) in Owerri educational zone I. The researcher adopted descriptive survey and pre-test, post test, control group designs. Simple random sampling and purposive sampling technique were used to select 20 NQETs, 10 experienced basic science teachers and 80 basic science students. The instruments used for data collection were; Researcher-structured checklist with observational rating scale on teachers’ knowledge of subject matter which was validated with reliability coefficient of 0.70 and the second instrument, classroom teaching competences and reflective ability questionnaire which was adopted and used. The third instrument was the Basic Science Achievement Test (BSAT) administered to students in both groups as post test. With the three instruments data were collected by observing the mentored teachers in classroom, while teaching students were post-tested. Data collected were analyzed, mean score and standard deviations were used to answer the research questions while the hypotheses were tested at 0.05 levels of significance. The findings of the study among others are; (1) That teachermentoring of NQETs enhanced teachers’ knowledge of subject matter, (2) That teacher-mentoring enhances professional competences and reflective abilities of NQETs. (3) Students taught by mentored teachers performed better than students taught by non – mentored teachers. Based on the findings, the researcher made some recommendations among which are:(1) that teacher training institutions should adopt teacher-mentoring to improve teachers knowledge of subject matter, professional competences and reflective abilities (2) that the State School Management Board (SEMB) and State Universal Education Board, should organize orientation on appropriate strategies of teacher mentoring for more experienced serving teachers (3) New qualified and employed teachers (NQTRs) should be assigned to teacher mentors of the same professional status (in the same institution) who will give them support and guidance required on their new profession.