Laser welding is a common technique for joining metals in many manufacturing industries. During welding, a weld gun traverses the interface of the parts to be joined causing them to melt, fuse, and solidify when the temperature decreases, thus joining the parts. Due to the heat input and the resulting melting and solidification, the parts deform causing residual distortion and residual stresses. To assure the geometrical and functional quality of the product, computational welding mechanics (CWM) is often employed in the design phase to predict the outcome of different design proposals. Furthermore, CWM can be used to design the welding process with the objective of assuring the quality of the weld. However, welding is a complex multiphysical process including the weld pool flow, microstructure dynamics, and structural mechanics. In a design process, it is typically not feasible, for example, to employ fluid simulation of the weld pool in order to predict deformation of a welded assembly, especially if a set of design proposals is under investigation. This is because of the high resolution needed for these fluid simulations in combination with challenges to couple fluid simulation with structural simulation. Instead, what is used is a heat source that emulates the heat input from the melt pool. An example of a heat source is the standard doubled ellipsoid. This heat source has been efficiently used for a large number of welding simulation. However, standard heat sources are typically not flexible enough to capture the fusion zone for deep keyhole mode laser welding. In this study, we presented a new heat source model for keyhole mode laser welding. In an industrial case study, a number of bead-on-plate welds have been employed to compare standard weld heat sources and develop the new heat source model. The proposed heat source is based on a combination of standard heat sources. From this study, it was concluded that the standard heat sources could not predict the observed melted zone for certain industrial application while the new heat source was able to do so. Therefore, the proposed heat source model can be employed to model keyhole mode laser welding, which enables welding simulation of a set of design proposals during the design process in a larger number of industrial cases.