I'd say the moment Mullen turned heads was when he led huge underdogs Combined Catholic HS to two consecutive wins in the Aus schoolboy selection trials.
Since that time he has represented Aus schoolboys two years running - the second year as captain - NSW u17s, NSW u19s, twice made the junior Kangaroos, the Prime Ministers 13, led the Knights to an SG Ball title and missed leading them to a Flegg title (he would have been the difference in the GF) because he was playing firstgrade at the time.
The selectors have been watching him very closely since he was 16.