Can I use a 2019 27" iMac as a display for a Mid 2014 Retina 13" MacBook Pro? The iMac is running Mojave, the MacBook is on High Sierra.

Will the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 adapter + a thunderbolt 2 cable work?
NO

The Apple documentation clearly states that this is not possible with iMacs newer than mid 2014 (iMacs with a Retina Display). 


  Note: Target Display Mode isn’t supported on iMac models with Retina display. Only iMac (27-inch, Late 2009), iMac (27-inch, Mid 2010), and iMac (Mid 2011 to Mid 2014) support Target Display Mode.


Apple Community states it as well.