Does Android Device Manager Work if the phone was reset? For this scenario let's assume the following:


  The phone is stolen and locked. The thief does not know the lock combination and cannot determine it.
  There is an external SD slot.
  The thief might already reseted my phone


In case it matters, the phone is a Sony Xperia z2
No, Android Device Manager will not work. It is so, as it does not have the data and information of its setting and accounts. But all of your data will not get in hand of thief.

Did you report it stolen with your carrier? They could report the ESN / IMEI as stolen so at the very least the thief probably wouldn't be able to sell it.