Walls graduated in late 1958 and, following service at sea in, was sent to the Britannia Royal Naval College in the United Kingdom for further training from April 1959. Promoted to acting sub-lieutenant, he returned to Australia in September 1960, served in and, then completed the Destroyer Gunnery Officer’s course at HMAS Cerberus in 1962. His next posting was to, during which he was promoted to lieutenant in February 1963. Following further specialist training at Cerberus later that year, Walls joined as part of the ship's commissioned crew in 1964. He remained on the Derwent for just over a year, which included service in the waters off Malaysia and Borneo as part of the Indonesia–Malaysia confrontation. In November 1965, Walls joined the crew of as the ship was commissioned into service in the United States. His time with Hobart, which was to last over two years, included a deployment to Vietnamese waters from March to September 1967. Hobart operated in gunfire support duties as part of the United States Seventh Fleet during this time, with Walls serving as an Air Intercept Controller; this included brief secondments to and. The six month tour saw Hobart fire over 10,000 rounds at 1,050 targets, with the ship itself being fired upon ten times but suffering no casualties. In recognition of this, Hobarts crew was recognised with the award of a United States Navy Unit Commendation. Walls returned to the United Kingdom from March 1968 for a three-year exchange with the Royal Navy. The posting occasioned service aboard the aircraft carrier and as a training officer at the Royal Naval Air Station Yeovilton. He was promoted lieutenant commander in February 1971 and, returning to Australian soon thereafter, was made direction officer on. This was followed by a period as a training officer at, before Walls was posted to the staff of the Flag Officer Commanding HM Australian Fleet in 1975. Made executive officer of the Perth in January 1977, he was promoted to commander in June that year. Walls' second posting to the Perth was a relatively brief one, as he was relocated to the Navy Office in Canberra for staff work from February 1978 for a period of almost four years. This was followed by appointment to his first ship command, the recently commissioned heavy-lift ship, in December 1981. Walls' period of command was marked by Tobruks first operational deployment. The ship left Brisbane on 15 February 1982 to transport eight UH-1 Iroquois helicopters of the Royal Australian Air Force, along with supporting stores, to join the Multinational Force and Observers in the Sinai Peninsula. On docking at Ashdod on 19 March, Tobruk became the first Australian warship to visit Israel. Walls and his crew arrived back in Brisbane on 30 April. In June the following year, Walls was made Commander Australian Amphibious Squadron and commanding officer of the naval base. On promotion to captain in June 1984, Walls returned to the Navy Office as Director of Naval Force Development. For his services in this role, he was appointed a Member of the Order of Australia in the Queen's Birthday Honours of June 1987. That same month, he was appointed to command the guided missile destroyer. Promoted to commodore in June 1988, Walls attended the National Defence College in New Delhi, India the following year, graduating with a Master of Defence Studies. Returning to Australia in 1990, he was appointed Director-General Naval Policy and Maritime Doctrine. He was promoted to rear admiral that June. Following a brief tenure as Deputy Chief of Naval Staff for eight months in 1991, Walls was appointed Maritime Commander Australia during a ceremony aboard his former command, HMAS Tobruk, on 7 November. In this role, he was responsible for the command of the Australian fleet. While Maritime Commander, Walls also served on the board of the Young Endeavour Youth Scheme. Advanced to an Officer of the Order of Australia in the Australia Day Honours of 1992, he was made Assistant Chief of Defence Force in 1994. Walls served in this role for a year, before he was promoted to vice admiral and appointed Vice Chief of the Defence Force on 20 April 1995 in succession to Lieutenant General John Baker, who had been appointed Chief of the Defence Force. Walls' tenure as VCDF coincided with an efficiency review into the Defence organisation, to which he was appointed to the senior review panel. Walls retired from the RAN in March 1997 after 42 years of service, and was succeeded as VCDF by Vice Admiral Chris Barrie.
Retirement
On his retirement from the RAN, Walls embarked on a corporate career in the defence industry. He was a director of the defence manufacturer Thales Underwater Systems from 1997 to 2003, on the board of the Australian defence contractor CEA Technologies from 1998 to 2000, chairman of Smart Shield from 1999 to 2001, and a director of the defence contractor ADI Limited from 1999 to 2003. Walls has also served on the Advisory Council for the Australian National University's Strategic and Defence Studies Centre. In 2002 he was recognised by the French government with appointment as a Knight of the National Order of Merit. A keen fly fisher and opera attendee, Walls lives in the Australian Capital Territory as of 2016.
