Residual stresses and distortions due to time dependent and localised heating imposed during fibre laser welding a 2.0 mm thick titanium alloy Ti-6Al-4V sheet were studied. Sequentially coupled thermo-metallurgical-mechanical simulations were performed to predict welding induced residual stresses and distortion in the fibre laser weld sample, and validated using an experimental database including weld pool geometry and temperature fields. Residual stress measurements were taken using X-ray and neutron diffraction techniques and distortion measurements were recorded using a coordinate measuring machine (CMM). The influence of thermally driven non-isothermal diffusional and diffusionless solid state phase transformations on welding residual stresses and distortions were considered in the numerical model. An internal state variables approach was used to represent the transformed volume fraction of different microstructural phases as a function of cooling rate and peak temperature, and the volumetric change due to temperature variations and phase transformations were calculated. In addition, post weld heat treatment (PWHT) as a method for reducing residual stresses was examined.