In his early career in the Foreign Service, Walter served in Thailand, New York, and Samoa.
In 1999, Walter became Secretary of the New Zealand Ministry of Foreign Affairs, until his retirement from the diplomatic service in 2002.
Walter was the chairman of the Environmental Risk Management Authority until 2008 and the chair of NZ On Air until 2012.
In the 2002 Queen's Birthday and Golden Jubilee Honours, Walter was appointed a Companion of the New Zealand Order of Merit, for services as Secretary of Foreign Affairs and Trade.
His son, Stephen Walter, is a lead negotiator on climate change for the New Zealand Ministry of Environment.