 He returned to Australia and Sydney's Northern Beaches in 1956, joining the Manly-Warringah Sea Eagles who played in the New South Wales Rugby League Premiership and became the cornerstone of their forward pack in the late 1950s.Ever an aggressive front-row forward, Mossop played in the Manly sides that lost to the St George Dragons in grand finals in 1957 and 1959.In the lead up to the 1959 Grand Final, rumours were circulating that Mossop was carrying a broken cheekbone.