Davies was born in Wellington but raised in Dunedin. His parents were Ina Amuri and Donald William Davies. He received his education at John McGlashan College in Dunedin. In 1965, he married Patricia "Trish" Helen Uren. They have one son and one daughter. Since the early 1960s, they have lived in Queenstown.
From 1958 to 1963, Davies was a high country musterer in Central Otago. In 1963, he purchased Wakatipu Transport Ltd and merged the company in 1967 with Northern Southland Transport Holdings Ltd, of which he is the chairman. He was director of Davies Rentals Ltd, which was the localAvis Car Rental franchise for Queenstown. He has invested into the tourism industry since the mid-1980s and credits the people who "voted out of the mayoralty" in the 1989 local elections for concentrating on the tourism industry more seriously. His first major purchase, in the mid-1980s, was the government's 40% share of Queenstown Airport. After the end of his political career, he purchased the guided walks at the Routeburn and Milford tracks, followed by The Hermitage Hotel in Mount Cook Village. Together with other entrepreneurs, he bought three ski fields off Air New Zealand: Coronet Peak, The Remarkables, and Mount Hutt. The companies are held through Trojan Holdings Ltd, with Davies the sole shareholder of that company. That company holds 40% of the shares of Bungy New Zealand Limited, with A. J. Hackett the public face of it. Trojan Holdings Ltd holds Aoraki/Mount Cook Alpine Village Limited, which in turn owns The Hermitage Hotel. Trojan Holdings Ltd holds NZSki Ltd, which in turn owns the ski fields. Tourism Milford Limited is also held by Trojan Holdings Ltd and trades as Ultimate Hikes. This company operates the guided walks on the Routeburn and Milford tracks. Trojan Holdings Ltd purchased the rights to the annual multisport eventCoast to Coast in 2013 from its founder, Robin Judkins. Davies owns Stratton House in Queenstown, which houses the town's casino marketed as Skycity Queenstown, and the Station building, which houses booking offices for many of the tourism ventures owned by Trojan Holdings Ltd and others. Davies was chairman of Queenstown Airport and chairman of the Coronet Peak ski field. In 2008, the new base building at Coronet Peak was named "JSD's Lodge at Coronet Peak", using Davies' initials in his honour.
