The 25-year-old unnamed maid filed a complaint with police last month accusing Dr Yonas Tegegn, the WHO's top representative in Thailand, and his wife of abusing her and forcing her to work without pay for nearly two years at a residence in Nonthaburi, a northern satellite city of Bangkok.
"After questioning witnesses we have decided there are no charges to be brought," investigating officer Colonel Mana Tienmaungpak told AFP.
Tegegn, also an Ethiopian national, has firmly denied any wrongdoing over the "baseless" allegations.
According to her lawyer, the maid said she had been promised a monthly wage of 3,000 baht ($92) -- roughly half Thailand's minimum wage for a five-day week.
The couple have said they paid her 6,000 baht a month and had agreed to keep it for her until the end of her contract this June, the police official added.
But Surapong Kongchantuk from the Lawyers Council of Thailand, said the maid worked without pay from July 2013 to March this year when she finally escaped her employers.
Legal proceedings will continue, said Surapong without detailing the next steps available for the maid, who is being helped by a local NGO.