Following his graduation Hallaj joined the Jacksonville University, Florida, as a lecturer and taught there until 1970.[2][3] Then he taught at the University of Jordan in Amman from 1970 to 1975.[3] During his tenure Hallaj served as dean of social sciences.[2] Next he joined Birzeit University on the West Bank in 1975 and became a full professor.[2] He was appointed vice president of Birzeit University and then was named as the first director of the Council for Higher Education in the West Bank and Gaza.[2] He taught at Birzeit University until 1981 when he went to the US for a study visit at Harvard University.[1] After the end of his study visit in 1983 he returned to Birzeit University, but the Israeli authorities did not grant him a work permit.[2] Therefore, he had to leave Palestine and settled in the US.[3]
Hallaj was the editor of a magazine entitled Palestine Perspectives between 1983 and 1991.[2] He was the director of the Center for Policy Analysis on Palestine from 1991 to 1994.[1] He was made a member of the Palestinian National Council in 1991.[3] He headed Palestinian delegation during the multilateral talks on refugees which had been initiated after the Madrid Conference in 1991.[1]
Hallaj was a board member of various Palestinian organizations such as the commissioners of the Palestinian Independent Commission for Citizens’ Rights.[1]
Hallaj published many articles on Palestinian affairs.[1] He was among the contributors of the 1988 book entitled Blaming the Victims: Spurious Scholarship and the Palestinian Question edited by Edward Said and Christopher Hitchens.[4]