Service-Learning (SL) has been defined as an experiential teaching methodology. Through SL, students participate in activities that benefit their community and enhance their learning experience. In the current study, Spanish as a second language (L2) and heritage language learners (HLLs) engaged in a SL project in which they translated English language zoo signage into Spanish. The objectives of this investigation were to determine whether SL mediates lexical knowledge, and to what extent learners’ linguistic background affects that knowledge. Participants included ten L2 learners and ten HLLs that translated signage in mixed dyads. A pre-test, a post-test, and a delayed post-test of target words were administered and analyzed via a paired sample t-test. Results indicate that vocabulary learning occurred as a result of the project. A qualitative analysis of learner reflections demonstrated the intricacies of learning. For instance, all learners gained an understanding of synonymic relations between low and high frequency words. With regard to HLLs, all four pedagogical goals outlined for heritage language pedagogy were met through the project, including language maintenance. The study concludes with pedagogical implications for the implementation of SL in the language classroom.