The club was formed in 1963 due to a high demand of junior rugby players searching for clubs on the northern peninsula. Being one of the most rapidly developing areas in Sydney at the time this subsequently led to the establishment in the 1962/1963 season of the Warringah Junior Rugby Union.
The Senior Club was formed to supplement the already established Warringah Junior Rugby Union and commenced playing in the Sydney second division in 1964. Its aims were to provide rugby for juniors, colts and grade players and to eventually join the 1st Division Championship competition. In 1970 the Club won the 2nd Division 1st grade Premiership and was promoted to the 1st Division for the 1971 season.
More affectionately known as the Rats or Ratties, the club took on this tag after experiencing many difficult years with many humiliating defeats. The players were reminded by Doug Leslie Snr (Rats of Tobruk), that to give up in the face of adversity is never an option. The tradition of the Rats of Tobruk, of bonding together and making commitments in the true spirit of mateship should always be remembered and will prevail just in the same way as the Desert Rats of Tobruk prevailed. From that time on the Club became known as the Green Rats and their long held friendship with the ‘Rats of Tobruk Veterans Association’ and their legacy is honoured by a memorial on the hill at the southern end of ‘Rat Park’.