Mr Thomas, who previously worked for Wood and Marathon Oil, had been Ithaca’s CEO for almost seven years.
He said he was “not free” to discuss his exit when contacted by Energy Voice.
Mr Dunnett was the head of Repsol Sinopec Resources UK (RSRUK) for five years before leaving the company at the end of March 2020.
He has also worked for Petrofac, Halliburton, Mobil North Sea and Shell during his 35 year career in the petroleum industry.
Mr. Dunnett has served as Chairman of the Board of MER UK Technology Leadership and a member of the Board of Oil and Gas UK and the Oil and Gas Technology Center.
While at RSRUK, he led a large-scale transformation of the business, dramatically reducing operating costs and bringing the business back into the dark.
He was also at the helm when RSRUK started production when it redeveloped the Montrose area to less than £ 2 billion in 2017.
Mr. Dunnett said he was pleased with the turnaround of RSRUK and would adopt a similar “philosophy” based on strong production performance and high safety standards in Ithaca.
He said he would spend the next two months developing a “battle plan” for Ithaca and that he was “optimistic” about the growth opportunities for the business.
Mr Dunnett also described Ithaca as ‘really dynamic’ and said he liked the ‘potential’ he saw in the expanded company following the integration of the UK’s North Sea portfolio from the US oil major. Chevron, acquired last year.
The deal, first announced in May 2019, gave Ithaca stakes in 10 production fields and increased its reserves by 150% to around 225 million barrels of oil equivalent (boe).
About 450 former Chevron employees, including 200 offshore, have made the move to Ithaca, giving the company a workforce of 500.
In February, Ithaca announced plans to sanction a number of new developments, including phase two of the Captain Enhanced Oil Recovery Project, a new crossing in the Grand Stella area called the Hurricane, and the exploration well Fotla near the Alba field.
Since then, oil and gas prices have fallen, forcing the company to cut capital spending and delay or postpone projects.
In June, it revealed its intention to lay off a quarter of its workforce ashore.