Born in Shanghai in 1947, Sum made her movie debut at age 13.
She appeared in more than 100 movies in Hong Kong and hosted more than 5,000 episodes of variety shows.
She also stared in the Singaporean English-language sit-com "Living with Lydia," which was broadcast in many Asian countries.
Donald Tsang, Hong Kong's chief executive, said "Hong Kong grew up with her laughter".
"She brought us a lot of joy," he said.
Sum is survived by ex-husband, actor Adam Cheng, and their daughter Joyce.