His deputy will be Southland Health Board chairman Paul Menzies.
These appointments will last until the new board is sworn in after the October elections.
Mr Millar, who previously served on the Southland board as a government appointee from 2002-07, said the two men had developed a close working relationship and that would continue under the new arrangement.
Mr Menzies has served on the Southland board since 2001 and both men were appointed to their respective chairman's roles last year. Mr Menzies said he considered there was quite a good "cross-pollination" between the two.
He was born in Dunedin and Mr Millar in Edendale.
There could be some people who still worried about "parochial things" but he did not expect it to be a problem.
Both men had indicated their willingness to chair the new board, which comes into being in May, but yesterday Mr Menzies indicated some relief at not getting the top job.
It would involve "a lot of time and a lot of travel.
"I'm trying to run a business".
Mr Millar said he was proud to lead the new organisation, with his biggest immediate challenge working out how to get through "the next nine months with a cast of thousands".
A major job would be integrating the services of the two boards, known as the provider arms.
This process will include input from a clinical advisory committee, the membership of which has yet to be confirmed.
Other major projects facing the new board were the transition to one single primary health organisation, due to be completed by July 1, and planning for rural hospital services.
Mr Millar paid tribute to the work of Susie Johnstone, who was appointed deputy chairwoman of both boards following the last election, and said he was delighted she would be continuing on the board.
Mr Ryall said all existing elected and appointed members would continue until the elections, along with the Southland board's crown monitor Stuart McLauchlan, who would fulfil that role on the new board.
Mrs Johnstone, who is from Otago, said she was " very comfortable" with the appointments and had not been expecting to get the deputy's job .
It was entirely appropriate that there should be leadership from both provinces.
The Otago District Health Board has its last full meeting tomorrow, but its advisory committees will meet for the last time in April.
The make-up of the hospital advisory committee has yet to be settled.
This is the only advisory committee where the boards do not have shared membership already.
The boards have agreed that in the first month of the new board, there will be a full meeting on May 6 in Invercargill, but rather than hold public advisory committee meetings, committee members will meet in workshops behind closed doors.
Asked why this was necessary for the two committees which already shared membership, Mr Millar said the committees had almost completed existing work programmes and the workshops would give members the opportunity to give undivided attention to what could be achieved between now and the end of the year.
One of the issues he expected to be raised was whether the disability support and community and public health advisory committees should meet separately.
During the term of the previous Otago board, these committees met jointly, although not all members served on both, but this practice was not followed in Southland and was abandoned this term when the boards moved to shared committee membership.
One of the criticisms of the joint approach had been the risk of disability issues being sidelined.
Mr Millar agreed there would be strongly held beliefs on both sides of the argument.
The new Southern board, comprising four elected representatives from Otago, three from Southland and four ministerial appointments, will be formed after the October local body elections.
