Mirza Ghulam Ahmad seems to have kept a diary, and most likely a “diary-writer”, since MGA couldn’t write very well, since he broke his arm as a teenager.  The 2nd Khalifa, Mirza Basheeruddin Mahmud Ahmad also admitted to keeping a diary writer as he was in court in 1953, he denied some interesting writings from his own diary and blamed an entire entry on having a new “diary-writer”.  MGA most likely used Mufti Muhammad Sadiq, Noorudin and many others.

A quotation that proves that MGA’s diary existed
On 25th April 1907, following page of Mirza Ghulam A Qadiani’s daily diary was published in Akhbar al-Badr, Qadian:

“Whatever has been written by Sanaullah, actually its foundation has been laid down by God. Once I meditated towards God about it and in the night I received this inspiration (ujeebo dawat adDaaee). For Sufis the big miracle is acceptance of prayers, rest is all its branches.” (Akhbar alBadr Qadian, 25th 1907, p.7 Column 3)