Wire arc additive manufacturing (WAAM) is envisaged to have a bright future in strengthening steel structures. This is evidenced through pioneering studies exploring the strengthening of hot-rolled steel I-section columns and I-section beams through the addition of material by WAAM, alongside investigations of residual stresses in WAAM-strengthened steel members. However, the residual stresses within WAAM-strengthened steel I-section beams have, hitherto, not been investigated. Thus, presently, the residual stress and residual distortion of three WAAM-strengthened steel I-section beams have been measured experimentally and simulated numerically. The widely used sectioning method was used for the measurement of residual stresses, with the results revealing that the original underlying residual stress distribution in the hot-rolled steel I-section was significantly altered after the addition of 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 lump-layer sequential thermal-mechanical finite element (FE) modelling approach was then used to simulate the residual stress and distortion of 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.