Use early 2009 20" iMac as external display for 2015 rMBP I wanted to use a mini display-port cable then use "target display mode" to make the iMac an external display, but I just read that early 2009 iMac's don't have target display mode as a feature.

Anybody knows of a solution?
This cannot be done with your particular iMac (2009 20")

Per the Target Display Mode System Requirements Page it specifically says:


  A supported iMac with OS X v10.6.1 or later.


The supported iMacs are:


  
  iMac (27-inch Late 2009)
  iMac (27-inch Mid 2010)
  iMac (Mid 2011)
  iMac (Mid 2012-Mid 2014)
  


However, there is an App that allows you to turn a spare Mac into an extended monitor.  It's called Air Display.  This is not my ideal solution as video intensive applications don't perform well with software solutions.

If you need a secondary monitor, you may have to get a dedicated monitor to meet your needs