The cleaner worked at the Lak Song MRT station from April 4 to 6 between 7am and 7pm.
On April 7 the patient moved to work at Tha Phra station before leaving at 12.30pm for feeling ill.
The MRTA said the cleaner visited a doctor the following day and was diagnosed with a common cold. The following week, on Thursday (April 15) the cleaner tested positive for Covid-19.
However, the cleaner confirmed to have stayed indoors when the symptoms first began.
MRTA has said all staff members who came in contact with the patient have been told to isolate themselves at home.