The cluster includes Bedfordshire, Cambridgeshire, Hertfordshire and Milton Keynes.
Thomas, currently chief executive of Cambridgeshire & Peterborough ICB, said: ‘This is a great chance for us to work together, focus on what truly matters to local people, and create a health and care system that's more connected, efficient, and built to last. Most importantly, it means making sure people can get the care they need, quickly and easily, when they need it most.
‘Each organisation has strong foundations and a shared commitment to improving care for our communities, and I'm proud to take on this role as we move towards joint working across this wider geography.'
Cluster chair Robin Porter added: ‘Jan is a highly experienced, innovative NHS leader with an impressive track record of achievement in Cambridgeshire and Peterborough.
‘I look forward to working with her over the coming years to deliver high-quality health and care services for people in Cambridgeshire, Peterborough, Bedfordshire, Luton, Milton Keynes and Hertfordshire.'
Clare Panniker, regional director for NHS England in the East of England, said: ‘Jan brings a wealth of experience of heath service leadership which will benefit patients across the area and the wider region.'