Simpson resumed his Law studies at Sydney University in 1919. He graduated with the degree of Bachelor of Laws and was admitted to the Bar in 1920. In his practice as a barrister he specialised in legal actions related to motor-vehicle accidents. Simpson was a member of the Nationalist Party of Australia and in the period 1922 – 1925 he nominated for three elections – both State and Federal – but without success. In 1925 he married Dorothy Margaret Peel Blackley with whom he had two sons.
In 1922 Simpson joined the Militia as a legal officer. In 1941 he was attached to the Second Australian Imperial Force with the rank of temporary Brigadier. He was sent to the Middle East where he served as deputy judge advocate-general of the 2nd A.I.F. He returned to Australia in June 1942 and was appointed deputy judge advocate-general at Land Headquarters in Melbourne.
National security
In September 1942 Simpson was appointed Director-General of Security in Australia. The director-general of security was Head of the CommonwealthSecurity Service and was based in Canberra. In this role Simpson reported to the Attorney-General, Herbert Evatt, who was his contemporary at Fort Street Boys’ High School and Sydney University. The Commonwealth Security Service investigated organisations and individuals considered likely to be subversive or actively opposed to the nation’s interests. It investigated espionage and sabotage. It vetted defence-force personnel and workers in defence-related industries. It controlled the issue of passports and visas. It was responsible for the security of airports and wharves, and factories engaged in manufacture of munitions and other items necessary for Australia’s war effort. It was responsible for radio security. Simpson was also responsible for identification of enemy aliens, and for their internment and release. In November 1944 Simpson was discharged from the Army and continued as a civilian in his role as director-general of security. In June 1945 he produced a report warning of the danger of the Communist Party of Australia.
1948 Australian National Airways DC-3 crash – In September 1948 a Douglas DC-3 named Lutana crashed en route from Brisbane to Sydney, killing all 13 people on board. Justice Simpson's report was critical of Australia's out-dated air navigation systems. As a result, he was subjected to strong criticism from members of the Government.
1950 Australian National Airways Douglas DC-4 crash – In June 1950 a Douglas DC-4 Skymaster named Amana crashed 22 minutes after take-off from Perth, killing all 29 people on board. It was Australia’s worst civil aviation accident.
Mr Justice Simpson retired from the Supreme Court in April 1960 suffering from Parkinson's disease. He died in Marrickville, a Sydney suburb, in November 1966.