The residual stress and residual distortion of three WAAM-strengthened steel I-section beams have been measured experimentally and simulated numerically in the present study. The widely used sectioning method was employed for the measurement of residual stresses, with the results revealing that the original underlying residual stress distribution in the studied hot-rolled steel I-section was significantly altered after the addition of the WAAM material, primarily owing to the heat input from the manufacturing process. Different residual stress patterns were observed for specimens with varying WAAM material arrangements. The overall bending distortion at the member level, also referred to as the pre-camber, and the local distortion at the cross-sectional level, were measured by means of 3D laser scanning. A lumped-layer sequential thermal–mechanical finite element (FE) modelling approach was then used to simulate the residual stresses and distortion within the examined specimens numerically. Good agreement was found between the FE results and the experimental observations. The measured residual stresses and the validated numerical approach can be employed in the design of WAAM-strengthened steel I-section beams and in the development of corresponding design provisions.