"Five years is about right for a leader in this type of role with the time commitment required and Ateed will continue to lead and positively contribute to Auckland's growth with a new chief at the helm," he said.
"I have applied myself as rigorously as I think anybody could to the job ... but I've always had a very clear view that there was a runway and at the end of those five years I would be ready to do something else."
O'Riley said he would become actively involved with several businesses in the technology and education sectors as a director following his resignation from the Ateed job, which paid between $380,000 and $390,000.
Ateed chairman David McConnell said O'Riley had been instrumental in turning the organisations into "an outstanding, progressive agency which Auckland and Auckland Council can be proud of.''
"Auckland has changed immeasurably in the past six years since amalgamation and we are proud of the prominent role that Ateed, on behalf of Auckland Council, has played in the region's economic development," he said.
O'Riley's tenure has not been without controversy.
Last year he pledged $500,000 to Duco Events to help fund Joseph Parker's world title fight - before looking at the event's business case and backing out.
He also got into hot water after it was revealed Ateed had spent half a million dollars on a global branding exercise for Auckland - "the Place Desired by Many".
Ateed was also criticised after creating a special contract in London for one of its senior executives, Grant Jenkins, at a cost to ratepayers of more than $230,000.
