Thunderbolt Display – How to Use with MacBook Pro via Mini DisplayPort

According to this source, you won't be able to do that. You have to have a Thunderbolt Mac and use a Thunderbolt cable to use a Thunderbolt iMac as an external display. This other source has a table that confirms it. Sorry!

Thunderbolt ports allow connectivity to Mini DisplayPort. Since your converter just plugs in and then an HDMI cable (of any length that you supply) is then plugged into your converter and into a display, it should work just fine provided there is no damage to either the HDMI cable or the converter itself. If the Mac isn't recognizing the display, check the HDMI cable or the display first. Try to test the converter on another Mac with Thunderbolt before you deem it dead but Thunderbolt does indeed work with Mini DisplayPort.