Collett said he would help Impress develop "high standards of journalism, ensuring freedom of expression, and safeguarding and protecting the public". His part-time role includes advising on the regulator's code of practice and offering counsel to its board.
He said: "I am delighted to join the code committee of Impress. I appreciate the importance of Impress's work as a rigorous, independent and effective arbiter of media standards in a digital age."
Collett will continue as managing director at Jonathan Collett PR, which he founded in 2013.
Prior to this, Collett was director of communications at the Press Complaints Commission between 2009 and 2013, and before this, between 2007 and 2009, he was head of comms at the Advertising Association.
Collett was also press secretary to former Conservative Party Leader Michael Howard between 2001 and 2005.
Impress, which is part-funded through charities financed by former Formula One boss Max Mosley, has 43 members, with pro-Corbyn online media outlet The Canary the most prominent.
While Impress is approved by the Press Recognition Panel, while rival IPSO is not, Impress does not have any of the major newspaper groups as members - many are part of IPSO, or else self-regulate.
The Press Recognition Panel was created in 2013 as a result of the Leveson Inquiry into press standards.