Aidan Heavey is quitting as chief executive officer of Tullow Oil but is staying on at the Anglo-Irish independent as chairman for up to two years.
The moves comes as current chairman Simon Thompson moves aside and quits the board after six years in the position.
Replacing Heavey as chief executive will be current chief operating officer Paul McDade.
Tullow said the switches have been made following an "internal and external process" to find a new boss, led by the nominations committee.
Heavey, who has been chief executive for 31 years, will leave the position at the upcoming annual general meeting on 26 April. He will then become non-executive director for a transition period of no more than two years.
"Aidan Heavey's appointment as non-executive chairman reflects the board's belief that, owing to the unique nature of Tullow's business and relationships across Africa, a phased transition in the leadership of the group is appropriate," a statement from the company read.
An analyst note from Barclays on the changes read: "In our opinion, the change indicates a continuation of the existing business strategy with Tullow now benefitting from a re-structured business."
McDade, who has served as chief operating officer for 12 years, said: "We have a world class business with high quality assets and strong relationships across Africa that underpin our business. We have begun the vital process of deleveraging our balance sheet and our focus will now move to returning the business to growth as the recovery in the sector develops."
The company this week agreed to farm down a 21.57% stake from its 33.33% interests in four blocks in Uganda to Total in a deal worth $900 million.
In a further change, Ann Grant is to retire as senior independent director at the AGM after nine years on the board. She will be replaced by Jeremy Wilson.
