Neven MacEwan played Junior Competition Rugby in Wellington in 1953 for Wellington Teachers College and was selected that year for the Wellington Junior Representative Team. The next year, 1954, he joined the Athletic Football Club and was selected for their senior side and became a regular member for the next fourteen seasons. He was selected for the Wellington Representative A Team in 1954 at the age of 20 and retained his position in the team for the next thirteen seasons. 

In 1955 he trialled for the ALL BLACKS and was selected as reserve for the 2nd Test against Australia at Dunedin. Neven made his debut for the ALL BLACKS the next year 1956 against the South African Springboks in the second test at Athletic Park, Wellington, as number 8. He became the first choice lock forward from 1957 until 1962. Selection of the ALL BLACKS then were made after Trial and North Island vs South Island, matches. This process gave players in minor Unions the opportunity to experience and develop their skills in games surrounded by the best players showing good form. Teams were always picked on form. 

Neven MacEwan played 52 matches for the ALL BLACKS including 20 tests. But he was dropped for the final test against the Wallabies in 1962, when for the first time Colin and Stan Meads were used together as locks.

That was to be the end of MacEwan at ALL BLACKS level. He was plagued by injuries for much of the 1963 season and missed his chance of making the tour of Britain and France.

While he received another ALL BLACK trial, his 16th, in 1965 the rest of MacEwan's career was spent almost entirely with Wellington.

Ironically, even though he was by now considered an international has-been, MacEwan produced in his latter years some outstanding rugby for Wellington.

He was part of a powerful pack which was the basis of Wellington's back to back wins over the Springboks in 1965 and the Lions in 1966. His last match for the province was in the dramatic 12-all Ranfurly Shield draw with Hawke's Bay.

In 1963/64 Neven MacEwan played a season of rugby in London playing for Blackheath Football Club as captain. He was also selected to play for the England Barbarians. 

Returning in 1964 to New Zealand Neven took on the responsibilities of Captain, Coach and Selector of the Athletic Rugby Club and won the Premier Competition – The Jubilee Cup – in 1965.

In 1965 Neven Organised and Captained Athletic RFC World Tour to England, Ireland, Hong Kong and Japan.