Cr Robertson won the role with seven votes against Moeraki ward Cr Kevin Malcolm who received five votes.
Cr Malcolm also missed out on the deputy position to Molyneux ward Cr Lloyd McCall (seven votes to five).
Cr Robertson, who has been on the council for 15 years, stood on a platform of cleaning up the region's waterways.
The council's water plan "must bravely recognise that business as usual is not good enough", she said in her candidate profile in the Otago Daily Times.
The vote comes after previous chairman Andrew Noone said yesterday he was no longer the best person for the role.
Cr Noone said the last term had been a difficult one for the council, and he believed an array of central government reforms and rising expectations would not ease pressure on the future chair.
Over the past three years, council had undergone a series of investigations, departures, setbacks and scrutiny.
One challenge new councillors would face was the results of a much-awaited report set to inform a decision on water take limits for the Manuherikia River.
Cr Noone said he would be supporting former deputy chairman Cr Malcolm and Cr Robertson for the top roles.
He believed Cr Malcolm had an understanding of how the organisation operated, and would know where the council's focus would need to be in the future.
Similarly, Cr Robertson would bring a wealth of knowledge and had a greater understanding of science, he said
