In 1965 he was in the team of young Rabbitohs who challenged St George in the Grand Final in front of a record breaking crowd of 78,065 at the Sydney Cricket Ground.

The advent of the four tackle rule in 1967 was tailor-made for the athletic, barrel-chested McCarthy and his coach Clive Churchill gave him license to stand wide in attack to make best use of his tank-like charges.

Bobby McCarthy first tasted representative football in 1964 scoring two tries for a New South Wales Colts side against a touring French team.

In the 1970 domestic Ashes series against Great Britain he appeared in the third Test and later that year in the World Cup in England where he played in all four of Australia's games including the victorious final.

He played three Tests in 1971–72 against New Zealand and appeared in one match in the 1972 World Cup in which campaign he was competing for the second row spots with his club teammates Paul Sait and Gary Stevens.

In 1974 Test series at home against Great Britain, McCarthy was selected in the deciding 3rd game in a veteran pack including his old Souths teammates Coote, Stevens, and John O'Neill and captained and coached by Langlands.

The old war-horses won the match 22–18 to retain the Ashes and McCarthy was one of the players who chaired Langlands from the field and who along with eight others were unknowingly making their farewell Test appearance.