Around 4,300 extra properties could be affected, taking the total in the borough to about 9,200.
Richmond Council says the move would take money out of the area and send it to the Treasury.
It says more than half the tax would come from four boroughs, including Richmond, Wandsworth and Kensington and Chelsea.
Cllr Gareth Roberts, Leader of Richmond Council, said: “Lowering the threshold from £2 million to £1.5 million would drag thousands more Richmond homes into an already unfair tax that would take money out of our borough and send it straight to the Treasury.
“Many of those homes are ordinary family houses in areas where prices have risen sharply over many years. Their owners are not responsible for the government’s budget shortfall, and they should not be seen as an easy target just because of their postcode.
“Our residents are already facing a £102 million funding hit over the next four years. A mansion tax, whether the threshold is £2 million or £1.5 million, would be another deeply unfair raid on our residents who are already being asked to shoulder far more than their fair share.
“The government must rethink this policy before it does real damage to our community.
“Richmond will not stay silent while our residents are used to plug a hole in the national finances.”
