Mayor de Blasio is losing his first deputy press secretary to the private sector, The Post has learned.

Austin Finan, who started working in Hizzoner’s press shop in February 2016, is leaving next week to become Mercury Public Affairs’ youngest-ever managing director at 32.

He had previously worked at Mercury from 2010 to 2016.

Finan most recently oversaw communications regarding the NYPD, MTA and Taxi and Limousine Commission for the mayor’s office.