He has been one of the most senior executives at ISBA, lobbying for advertisers’ interests in Westminster and Whitehall and with European regulators.
Twinn, a Conservative MP from 1983 to 1997, said: "I have had an amazing 18 years working for our UK advertisers, helping to steer a path between attacks on advertising and the very real need for responsible, consumer-facing, rules."
Phil Smith, ISBA’s new director-general, said: "ISBA and our members have benefited from Ian’s tremendous political acumen and nous as well as his serious negotiating skills.
"ISBA members and staff as well as the whole sector will miss him and wish him every success in this next stage of life."
Twinn is also the regional chairman of the Conservatives in London and served as MEP for London from 2003 to 2004.
He is the third long-serving member from ISBA to depart, following Mike Hughes who stepped down as director-general after 10 years and Bob Wootton who left as director of advertising and media after 20 years.
Smith told ISBA’s annual conference last week that he has been appointed with a "mandate for change from our members" and promised to "modernise" the trade body.