In recent years, there has been a growth in the number of evidence-based programmes designed to act directly on the factors that predispose or precipitate the onset of disorders or symptoms, rather than designing specific interventions to deal with each particular problem or disorder. The development of the Universal Strengthening Families Programme 11-14 (SFP 11-14) is one such example, where parents and adolescents are trained through the acquisition of strategic skills. The aim of this study is to assess the effectiveness of SFP 11-14 in tackling internalizing and externalizing symptoms in adolescents from 11 to 14 years old. A quasi-experimental design was used with a control group, based on pre-test and post-test assessments and a follow-up. 289 adolescents and 353 parents took part, with 16 experimental groups and 17 control groups. From an analysis of variance and post-hoc Tukey-B tests, the effectiveness of SFP 11-14 in dealing with internalizing and externalizing symptoms was explored, confirming the efficacy of its short multicomponent (6-session) structure at the evaluation point (post-test1) and 6-month follow-up (post-test2). The results confirm that short preventive multicomponent programmes can prevent externalizing and internalizing symptoms in early adolescence. Improved family dynamics and relationships act as protective factors against possible mental disorders and against internalizing and externalizing symptoms. However, in future research, a specific assessment should be made of the effectiveness of each component in order to reinforce the more productive input, as well as conducting longitudinal evaluations so as to confirm long-term outcomes.