Light-emitting diode (LED) drivers are widely regarded as the weakest link in the solid-state lighting systems. This paper proposes an improved thermal modelling process for the mission profile based lifetime prediction of reliability critical components in a LED driver for the outdoor lighting application. A converter-level finite element simulation (FEM) simulation is carried out to obtain the ambient temperature of electrolytic capacitors and power MOSFETs used in the LED driver, which takes into account the impact of the driver enclosure and the thermal coupling among different components. Therefore, the proposed method bridges the link between the global ambient temperature profile outside of the enclosure and the local ambient temperature profiles of the components of interest inside the driver. A quantitative comparison of the estimated annual lifetime consumptions of MOSFETs and capacitors are given based on the proposed thermal modelling process, and the datasheet thermal impedance models and the global ambient temperature.