Fenwicke was born in Walcha, New South Wales and attended primary school there.
After making a success of schoolboy rugby at The King's School, Parramatta, Peter Fenwicke returned to Walcha in country New South Wales and at 17 in 1951, he was made captain of the Walcha Rugby Club, a position he held until his retirement in 1963.
[2] Fenwicke was first selected for New South Wales against the All Blacks in 1957 and he made his representative debut for Australia in a Test against those same NZ visitors in Sydney.
[3] He was selected for the 1957–58 Australia rugby union tour of Britain, Ireland and France and played in nineteen matches on tour including Tests against Wales, Ireland and England From 1958 Fenwicke was a regular starter in the New South Wales rugby union team, he captained the side from 1959 and there were five straights wins under his leadership against Queensland and the visiting British & Irish Lions.
His two matches as national captain were against the British & Irish Lions during their tour to Australia in 1959.