Abstract: A mullion assembly is disclosed comprising a pair of complementary mullion clip portions, each configured to be self retaining or frictionally engaged to a pair of window frames. Each mullion clip portion is configured to snap into corresponding recesses in the first or second window frame without the need for additional fasteners. The clip portions include integrally formed thermal barrier tabs to block airflow between the window frames. The clip portions may also have self-aligning features such as correspondingly shaped projections and recesses formed in abutting surfaces of the clip halves to facilitate alignment and fit-up of the window frames. The mullion clips may also have barbs for engaging interior and exterior snap on covers that are used to cover the gap between the first and second window frames and form a finished outer appearance.