Dr Ko Siew Lan graduated with a Bachelor of Dental Surgery (BDS) in 1971. She then went on to work in Singapore General Hospital as a dental officer in all the dental departments on a rotation basis for a year. Following which, she moved to the School Dental Service treating children and teenagers up to 18 years old from 1972 to 1974. Then, she was in charge of six school clinics.
From 1975 to 1976, Dr Ko was in England working in a private practice. Back in Singapore, Dr Ko was a lecturer in the conservative department of the University of Singapore, Dental Faculty. She also ran the Preventive and Dental Public Health Division there from 1976 to 1978.
Since 1979, Dr Ko has been in private practice here in Ko Djeng performing general dentistry with a special interest in paediatric dentistry and crown and bridge prosthodontics.