Both civilizations flourished in similar periods and shared many geographical, cultural, and political elements in common.
The ancient Mesopotamian civilizations were located between the Tigris and Euphrates rivers, while Harappa and Mohenjo-Daro were both located in the valley of the Indus river. Thus both civilizations were highly dependent on rivers for irrigation.
Both civilizations were agricultural (as opposed to hunter-gatherer) and required large groups of people to coordinate complex schemes of irrigation in order to produce sufficient food.
Both of these civilizations were highly urbanized, with sophisticated transport and economic networks enabling them to raise food in the surrounding areas in order to support wealthy cities with advanced crafts, artistic production, sophisticated religions, technology, and government bureaucracies.
Both of the civilizations had some form of writing. Although Mesopotamian writing has been deciphered, the Indus script, which evolved in the same period and was in widespread use, has not been deciphered, meaning that we know much less about the Indus civilizations than we do about Mesopotamia.
Both had polytheistic religions, although less is known of the Indus one due to the lack of deciphered written texts.