A government order regarding the appointment of Muthkumaraswamy was issued by the Public (Law Officers) department.
The notification came hours after Somayji resigned, bringing an end to his stint as the Advocate General since March 2013.
Muthukumaraswamy has served as Additional Advocate General of the state during the previous AIADMK regime in 2001-06.
He brings with him rich experience in civil, writ, service and service jurisprudence.