Y. Bhg. Tan Sri Datuk Zainun Binti Ali ("Tan Sri Zainun") was appointed as Legal Officer of the Ministry of Labour and Attorney General's Chambers in 1976. She was appointed as Magistrate in Malacca in January 1979 and became President of the Session Court in Johor Bahru in December 1981.

In the middle to late 1980s, she was Senior Federal Counsel to several Government units, including the Ministry of Defence in November 1985 and the Prime Minister's Department in March 1988.

She was appointed Chief Registrar of the Federal Court of Malaysia in November 1994 and Judicial Commissioner of the High Court of Malaya, Kuala Lumpur in August 1996.

She served as High Court Judge in Shah Alam, Johor and Kuala Lumpur from 1998 to 2001. In 2006, she was elevated to be Court of Appeal Judge and served until 2012. She was appointed as Federal Court Judge on 4 April 2012.

Tan Sri Zainun retired from the Malaysian Judiciary in October 2018. She was appointed as one of ten members of the United Nations Office on Drug and Crime Advisory Board of the Global Judiciary Integrity Network on April 2018 which is established to promote judicial integrity amongst Judges and stakeholders. She is currently the deputy chairman of the Enforcement Agency Integrity Commission (EAIC) and a member of the Advisory Tribunal for the Islamic Development Bank based in Jeddah, Saudi Arabia. She is also a Consultant in the legal firm of Zain & Co.