Liberal Democrat Dinah Keal was voted in as the new chairman of the authority's Policy and Resources committee when the council met last week.
Ryedale councillors this year chose not to elect a councillor to the official role of leader which means the chairman of the Policy and Resources committee represents the authority in situations previously reserved for the leader.
Cllr Keal replaces Liberal John Clark, who had represented the Cropton ward on the district council since 2003.
Mr Clark died last month after he was admitted to hospital with breathing difficulties.
Cllr Keal, who had previously been vice-chair of Policy and Resources, was unopposed for the role at last week's full council meeting.
She said: "It is a privilege to be given the role of chairman of a committee that has such an important role in the running of the council and its direction of travel.
"Ryedale District Council has a huge challenge on its hands as a result of Local Government Reorganisation and the fact that it will cease to exist in its current form in 18 months time.
"There is a huge amount to do. Our aim in this time is to continue to deliver for residents in Ryedale during the reorganisation period and to help to bring significant projects that are now underway to fruition, such as developing a new Livestock Market, new business units, enabling community facilities in Pickering and the ongoing transformation of the Milton Rooms."
Independent Group leader Lindsay Burr will replace Cllr Keal as vice-chair.