The main purpose of this study was to examine the influence of career guidance services provided in public secondary schools on students’ choices of future professional training in Magu District, Mwanza- Tanzania. This study was guided by three specific objectives which were to explore the factors which make secondary school leavers enter into unplanned professional training, determine whether career guidance services in secondary schools enable the students to choose their future professional training and examine the challenges facing teachers in the delivery career guidance services to public secondary school students in Magu District. The study was guided by Social Cognitive Career Theory (SCCT) that was developed by Lent, Brown, and Hackett in 1994. The study adopted mixed research approach and convergent parallel research design. The sample size was 152 participants. Quantitative data were collected through questionnaires and analyzed using descriptive statistics with the helper of Statistical Package for Social Science (SPSS) version 21 and presented in frequency, percentages, graphs and charts. Qualitative data were collected through interview and analyzed thematically by creating themes. The study revealed that career guidance service that was provided in secondary schools was ineffective; hence, making secondary school leavers to join unplanned professions and training since it did not contribute to the students’ choices of future professional training. The provision of career guidance service is ineffective due to the challenges such as shortage of materials, lack of cooperation, lack of professional training on career guidance and lack of qualified career guidance service personnel. The government should ensure provision of career guidance services training to teachers.