Rescue efforts continued through the night, with workers digging through mud and debris to find survivors.
Indian Prime Minister Narendra Modi offered his condolences to the families of those killed.
In the 24 hours ending Monday morning local time, Mumbai airport recorded more than 250 millimeters (9.8 inches) of rainfall, according to data analyzed by CNN Weather.
Mumbai, a city of 12 million people, regularly experiences heavy rainfall during the July-September monsoon season. The rain often triggers building collapses — especially in poorer districts known to house illegal or poorly built dwellings.