I am employed by a Hong Kong employer but assigned to work in the Mainland. Am I covered by the MPF system during my working period in the Mainland?

Yes. According to the MPF Schemes Ordinance, the MPF system covers employees in Hong Kong or those employed by a Hong Kong employer. As long as your employment contract is set up based on the Employment Ordinance of Hong Kong, your employer is required to enroll you in an MPF scheme no matter where you are deployed to work.