Justice G.P. Mittal joined Delhi Judicial Service in May, 1977. He held various posts including Metropolitan Magistrate, Senior Civil Judge, Rent Controller, Additional Sessions Judge, Special Judge of CBI etc. In the year 2008, he was posted as the Principal Secretary (Law & Judicial), Govt. of NCT, Delhi and thereafter in the year 2009 he became the District and Sessions Judge for the City State of NCT of Delhi. He was elevated as the Judge of Delhi High Court in the year 2010 and joined as Member of Competition Commission of India in July, 2015.