Vedic origin

The Kayastha trace their genealogy from Adi Purush Shri Chitraguptaji Maharaj. It is said that after Lord Brahma had created the four Varnas (Brahmins, Kshatriyas, Vaishyas and Shudras), Yama (Dharamraj) requested Lord Brahma to help him record the deeds, both good and evil, of men, and administer justice.
Lord Brahma went into meditation for 11000 years and when he opened his eyes he saw a man holding a pen and ink-pot in his hands, as well as a sword girdled to his waist. Lord Brahma spoke: Thou hast been created from my body (Kaya), therefore shall thy progeny be known as the Kayasthas. Thou hast been conceived in my mind (Chitra) and in secrecy (gupta), thy name shall also be Chitragupta. Brahma then enjoined him to dispense justice and punish those who violated the dharma. Thus, the Kayasthas were accorded a dual caste, Brahmin/Kshatriya.SEE NOTES
In the legends of Shree Chitraguptaji Maharaj, he is referred to as the greatest King, while the rest are "Rajakas," or little kings.
चित्र इद राजा राजका इदन्यके यके सरस्वतीमनु ।
पर्जन्य इव ततनद धि वर्ष्ट्या सहस्रमयुता ददत ॥ RIG VEDA 8/21/18
In the Garud Puran, Chitragupta is hailed as the first man to give the script.
"Chitragupta namastubhyam vedaksaradatre"
(Obeisance to Chitragupta, the giver of letters)
The Rig Veda mentions an invocation to be made to Chitragupta before offering sacrifice. There is also a special invocation to Chitragupta as Dharmraj (Lord of Justice) to be made at the performance of shradh or other rituals.
"Om tat purushaya vidmahe Chitragupta dhimahi tena lekha prachodayata."
The priests also pay reverence to Shri Chitragupta :
"Yamam Dharmarajya Chitraguptaya vain namah."'