Improving students’ conceptual understanding and problem-solving performance in electricity is a central goal of secondary school physics education, as it enables them to apply theories and knowledge of electrical phenomena to various real-life situations. Success in grasping these abstract concepts and solving electricity-related problems strongly depend on students' ability to use multiple representations effectively. However, many secondary school students find it challenging to explain a concept in different ways, as it is a novel approach for them. The process of translation between representations and making the appropriate math-physics connections falls within students’ zone of proximal development, where they require guidance and support. This study examined the impact of using multiple representations-based instruction combined with formative assessment practice on secondary school students’ conceptual understanding and problem-solving performance in electricity at Dagmawi Tewodros Secondary School in Debre Tabor, Ethiopia. To achieve this goal, an explanatory sequential mixed approach employing a pre-test, post-test, control group quasi-experimental research design was utilized. The study was conducted with 101 Grade 11 students enrolled in two randomly selected intact classes. Students' conceptual understanding of electricity was assessed using an adapted Conceptual Test of Electricity (CTE), while problem-solving skills were evaluated with four electricity problems scored using an adapted Problem-Solving Performance Rubric (PSPR) that emphasizes best practices in problem-solving. Data analysis included independent t- test, pairwise t-tests and Multiple Analysis of Covariance (MANCOVA). Qualitative data, gathered through think-aloud interviews and semi-structured interview were analyzed thematically to understand students' reasoning in their problem-solving approaches and explore the impact of the intervention on students' conceptual understanding and problem-solving approaches. The findings of the study revealed that instruction utilizing multiple representations coupled with formative assessment practices, was significantly more effective in enhancing students' conceptual understanding and problem-solving performance in electricity compared to conventional lecture-based instruction. Thematic analysis of think-aloud interviews showed low achievers often relied on formula-based, trial-and-error approaches with limited conceptual understanding and poor representational skills, while high achievers displayed systematic reasoning, strong vii visual representation use, and self-monitoring. Semi-structured interviews further indicated that, the reformed instructional approach helped students visualize abstract concepts, foster connections between different ideas, activate prior knowledge, and receive valuable feedback and support. Thus, by offering various ways to understand concepts and guiding learning through formative assessment, educators can significantly enhance students' conceptual understanding and problem-solving, particularly in complex topics like electricity.