Khoo Swee Seng started his working career with Tenaga Nasional Berhad in 1976. In 1979, he joined Malaysia Airlines as its security personnel with his main areas of work in anti-hijacking and warehousing. Khoo Swee Seng left the airline industry after in it for 15 years and tried his hand in retailing. He was in retail industry for 4 ½ years, during which he served as floor manager for Makro Cash & Carry, Seberang Prai for 3 years.

Khoo Swee Seng left the retail industry to embark on a career in the legal field. He worked for Mohamed Illiayas & Co for 9 ½ years as its office manager. Subsequently, Khoo Swee Seng left Mohamed Illiayas & Co and joined Sheila Hussain Vijay & Partners as its office manager.