Jallow was called to the bar in the Gambia and Nigeria in 1977. He was enrolled as a barrister and solicitor of the supreme courts of the Gambia and Nigeria. Jallow worked as a state prosecutor at the Attorney General's Chambers in the Gambia from 1977 to 1982 and was principal state counsel for a period of time. He also served as acting Registrar General in charge of the registration of companies, patents, trademarks, and so on. At this time, he also worked as a legal expert for the Organisation of African Unity and held to draft the African Charter on Human and Peoples' Rights which was adopted in 1981. Jallow was appointed as Solicitor General in 1982, and as Attorney General and Minister of Justice in July 1984. He was removed from this role following Yahya Jammeh's coup d'état in July 1994. He was also part of an expert group that helped draw up the African Charter on Human and Peoples' Rights, which came in force in 1986. Between 1989 and 1994, he served as the chairman of Banjul-based African Centre for Democracy and Human Rights Studies. In 1997, he was found guilty by the Public Assets and Properties Recovery Commission, and in 2001, he was banned from holding public office for five years. From December 1998 to July 2002, he served as a justice of the Supreme Court of the Gambia. He also carried out a judicial evaluation of the International Criminal Tribunal for Rwanda and the International Criminal Tribunal for the former Yugoslavia. He worked for the Commonwealth of Nations as chair of their Governmental Working Group of Experts in Human Rights and as a member of the Commonwealth Arbitral Tribunal. In July 2002, he was suddenly dismissed from the government with no reason given but it was likely linked to a supreme court ruling in the Ousman Sabally case.
UN judge
In 2002, he was appointed as a judge of the Appeals Chamber of the Special Court for Sierra Leone and served until 2003. In 2003, Kofi Annan, the Secretary-General of the United Nations, nominated Jallow as the Prosecutor of the International Criminal Tribunal for Rwanda. He was confirmed in this role by the United Nations Security Council, succeeding Carla Del Ponte on 15 September 2003. Jallow became the first ICTR Prosecutor to not also be the Prosecutor of the International Criminal Tribunal for the former Yugoslavia. His mandate was renewed by the UN Security Council in 2007 and 2011. On 1 March 2012, Jallow was also appointed as the Prosecutor of the Mechanism for International Criminal Tribunals, to serve for a four-year term. He was a member of the Independent Review Panel on UN Response to Allegations of Sexual Abuse by Foreign Military Forces in the Central African Republic, alongside Marie Deschamps and Yasmin Sooka. Upon the conclusion of his term, he was praised by Secretary-General Ban Ki-moon as "instrumental to the successful fulfilment of the mandate of the and the efficient conduct of the work of the Office of the Prosecutor". In 2015, Charles Chernor Jalloh of Florida International University and Alhagi Marong of the United Nations International Criminal Tribunal for Rwanda praised Jallow's service in a research paper.
On 15 February 2017, Jallow was sworn-in as the Chief Justice of the Gambia after his appointment by newly elected President of the Gambia, Adama Barrow. At the ceremony, he said "I have heard on a number of occasions the president reiterate his commitment for the judiciary and to its effectiveness. This declaration coming from the Office of the President to maintain the independence of the Judiciary is, indeed, very assuring and an excellent starting point for a new Chief Justice."
