In 1949 he married Mary Guy and they had four children: Thea, Hanna, Christopher and Tareq. He soon accepted an invitation from the United Nations to work in its Statistical Office in Lake Success then New York. In addition to his position in the United Nations, he became part-time visiting lecturer in the Mathematical Statistics Department at Columbia University. Salem finally visited his home in Israel in 1952. In 1953 he returned to AUB as Associate Professor of Economics. Between 1955 and 1958 he was appointed Professor and Chairman of Mathematics Department. Between 1958 and 1963 he became United Nations Food and Agriculture Organization Regional Statistician for the Near East. Between 1961 and 1970 Salem became Chief of the FAO Trade Prices Branch in Rome. Between 1970 and 1972 he was Director and UN Project Manager at the Institute of Statistics and Applied Economics, Makerere University, Kampala, Uganda. In 1972 he returned to Rome to become Head of the FAO Methodology Group – Statistical Development Service until 1974, and Chief of the Service 1975-1981. Work consisted mainly in assisting developing countries in the improvement of their food and agricultural statistics through national, regional and international statistical projects. In parallel, he acted in Baghdad as UN Project Manager/Chief Advisor to the Arab Institute for Training and Research in Statistics. In 1981 Salem resigned from his position at FAO and moved to England where his children lived. However, he continued to work as an expert and as the head of several scientific missions by the UN, and provided consulted at many countries, such as:
1981-1987: Statistics Advisor, Arab Fund for Social Economic Development, Kuwait
Prof. Khamis contributed tens of scientific research papers in Statistics and Mathematics. Specifically, he contributed to Sampling theory and the tabulation of the Incomplete gamma function, where he wrote the book “Tables of the Incomplete Gamma Function Ratio”. He contributed in the field of Index Number Theory through a series of papers starting 1972. He developed what became known as, and “indelibly imprinted on all the recent work on international comparisons of prices, real incomes, output and productivity”, the Geary-Khamis Method of Computing Purchase Power Parity of Currencies and the Geary-Khamis dollar used to compare different economies. In addition to his professional work as an international statistician, Prof. Khamis helped many students from Arab countries and many other nationalities to receive scholarships, supporting many students in developing their careers. He was also known for his strong support for all the underprivileged in the world, and for his continuous support of the Palestinian refugee camps and the Palestinian cause.