A fusion power plant requires not only the control of the high energy plasma but also advanced techniques for maintenance and assembly in order to generate electricity consistently and safely. Laser welding is a promising technique for cutting and joining pipes and in-vessel components made of Eurofer97, a European baseline structural material. However, the substantial residual strain, usually induced during post weld cooling, degrades the mechanical properties and reduces the lifespan of engineering components. Establishing the underpinning mechanistic connection between residual strain, microstructural change, and tensile behaviour is a critical aspect of the lifetime assessment of critical engineering components. Here, the heterogeneous strain evolution in laser-welded Eurofer97 joint is quantitatively evaluated using in situ neutron diffraction at the lattice scale, nanoindentation at the microscale and digital image correlation (DIC) at a macroscale. The residual lattice strain in the loading direction is characterised via neutron diffraction and validated using a plasma-focused ion beam (PFIB-DIC) ring-core method. Superimposing the microstructural strengthening, the highest residual tensile strain accelerates the accumulation of tensile deformation around the fusion line (FZ/HAZ interface), whereas residual compressive strain hinders the tensile strain evolution around the heat-affected zone and the base material interfaces, increasing the localised yield strength to 506 MPa. Residual strain is the primary strengthening mechanism during the initial deformation stage, although the microstructural strengthening then dominates as deformation increases. This work reveals the critical role of residual strain and microstructural effects on tensile behaviour, and the results provide insight into managing structural integrity and developing predictive tools for lifetime assessment.