Ed Casey will leave the firm at the end of the year to join a non-competitor in the US.
Rupert Soames, chief executive, said: “Whilst we are very sad that Ed is leaving, we completely understand that after four years of weekly commuting across the Atlantic, he would like to have a job closer to home.
“I would like to place on record our sincere gratitude for all he has done for Serco over the last 12 years, and in particular helping to lead the company through the critical early years of the transformation of our business.”
Earlier this year, Serco reached financial close on the contract to deliver the New Grafton Correctional Centre in Australia.