This paper aims to develop and compare the integrated circuit conducted immunity (ICIM-CI) models of two different voltage regulator ICs with similar functionality (i.e., UA78L05 and L78L05) under the combined influence of two different electrical overstress (i.e., 9 and 12 V) and high temperature (i.e., 70–110 °C) stresses ageing conditions for a total stress duration of 950 hours. Step-stress accelerated degradation tests (SSADT) are performed on two samples of UA78L05 (i.e., T1 and T2) and L78L08 (i.e., S1 and S2) manufactured by Texas Instrument and STMicroelectronics, respectively. Direct power injection (DPI) test was conducted in nominal conditions (i.e., 7.5 V and 25 °C) on these selected ICs according to IEC 62132-4 standard before and after the accelerated ageing test. For the studied cases, it was found that the passive distribution network (PDN) models extracted before and after ageing test for the ICIM-CI model of UA78L05 and L78L05 remained unaffected. However, the ageing process affected the internal behavioral (IB) block of the ICIM-CI model significantly due to noticeable reduction in the conducted immunity level characterized by the extracted IB look-up table data in terms of power transmitted and frequency. Both fresh and aged samples of L78L05 demonstrated higher conducted immunity profile compared to those of UA78L05.