Mehlis' spokesperson, who asked not to be named, said on Tuesday the date for Mehlis' departure was not yet determined, but added that the German prosecutor had made it clear when he accepted the job that he would stay on for only six or seven months.
The investigation into al-Hariri's 14 February murder was launched in June, and Mehlis is expected to submit his findings to the UN Security Council on 12 December.
Lebanon asked the world body last week to extend the inquiry's mandate for six months.
"When he came, he said he would be committed for six or seven months. Therefore his mandate is about to end," the spokesperson said in Beirut.
"He will certainly be available if required but not as a full-time commissioner."
Lebanese political sources have said Mehlis is likely to remain in charge of the inquiry beyond mid-December.
Meanwhile, the questioning of top Syrian intelligence officials by UN investigators over the assassination of al-Hariri earlier this year has been adjourned, official sources in Vienna said on Tuesday.
Questioning will resume on Wednesday at the UN Office in Vienna, an official source who refused to be named said, without adding any detail.
The grilling of five top Syrian intelligence officials began in secrecy on Monday in the Austrian capital.