In response to concerns about declining student motivations in secondary school mathematics, especially for girls, this 1-year mixed-methods longitudinal study investigated the effects of embedding Grade 7 mathematics in scientific real-world problems through four projects at one intervention school. These projects were implemented during mathematics and science lessons with the same teacher for both subjects. The aim of the projects was to highlight mathematics ‘utility value’ through its real-world relevance and to examine impacts on student learning and motivations, and teacher self-efficacy. Data from another school that did not implement the projects enabled comparisons between student and teacher outcomes. Data collection at both schools included student and teacher questionnaires and student achievement tests at the beginning and end of the year. Classroom observations were conducted, with additional questionnaires and interviews with selected students and their teachers after each project. Students’ outcomes were compared by gender and achievement groups, and parallel data collected for science to identify any discernible corresponding benefits. More favourable changes at the intervention school highlighted the positive impact of the projects. There were greater achievement gains in science topics taught through the projects. Students reported increases in personal relevance for mathematics and science, preparedness to apply 21st-century skills in mathematics, and mathematics-related career aspirations for boys and girls. Teachers reported increased teaching self-efficacy, although reported challenges in project implementation, with nearly half teaching mathematics out of field. The approach taken to promote students’ mathematics utility value through the projects extends the literature on utility-value interventions in secondary mathematics. Findings suggest classroom and policy implications for educators, particularly for motivating girls’ in mathematics.