Like most Asian actors in non-Asian countries, he played primarily small, stereotypical roles, though he rose quickly to familiarity, if not fame, in a number of films.

His stern features led him to be a favorite movie villain, and the outbreak of World War II gave him greater prominence in roles as vicious Japanese soldiers in such successful pictures as The Purple Heart (1944) and God Is My Co-Pilot (1945).

In the film The Purple Heart he plays a Japanese Imperial Army general who commits suicide because he cannot break down the American prisoners.

In 1974 he appeared as the Thai billionaire tycoon Hai Fat in the James Bond film The Man with the Golden Gun, opposite Roger Moore and Christopher Lee.

Loo was also a teacher of Shaolin monks in three episodes of the 1972–1975 hit TV series Kung Fu and made a further three appearances as a different character.