Siew Cheong graduated from Bristol University in 1988. He was admitted to the Bar of England and Wales in July 1989 as a Barrister-at-Law of Grays Inn. He was thereafter admitted to the Malaysian Bar in June 1990.

Siew Cheong was employed with Messrs Shearn Delamore & Co. as a legal assistant from June 1990 after completing his pupillage there, and later he joined Messrs Kumar Jaspal Quah & Aishah in 1993 and was a partner of the firm from 1997 to 2007.