During the 1957 Five Nations Championship, Prosser was rewarded with a position in the Welsh squad that would see him represent his country in the competition for the next three years without missing a game.
In 1959, Prosser was one of nine Welsh players to be selected to play in Ronnie Dawson's British Lions team when they toured Australia and New Zealand.
[6] He also suffered an injury early in the tour, but after his recovery, he played with his normal good form, impressing in the game against North Auckland.
During Prosser's time at Pontypool, he oversaw the 'Pontypool Front Row', made up of Graham Price, Bobby Windsor and Charlie Faulkner, the three players represented Wales on 19 occasions.
[9] Prosser coached the Pontypool first team for 18 years until 1987, during which they were unofficial Welsh club champions on five occasions including a hat trick between 1983–1986.