AMs praised Alan Murray for stabilising the Welsh Ambulance Services NHS Trust as its chief executive after a turbulent period.
He was appointed in 2006 – the third person to fill the post that year – at a time when the organisation's reputation was taking a battering.
Mr Murray said wide-ranging changes had significantly improved the service since he took up the reins.
The first university-educated paramedics will graduate in August and specialist practitioners were treating people at home, keeping them out of hospital, he said.
He added: "While strong foundations have been laid for the future, a lot of hard work remains to be done.
"I am proud of what our staff have achieved, but I believe that I have taken the trust as far as I can.
"I want to thank everybody who works for the Welsh ambulance - service, staff and volunteers - for their hard work and dedication. They fully deserve the confidence and admiration of the people of Wales."
The service has been repeatedly criticised for poor performance.
The most recent figures show that in January 58.5 per cent of responses to immediately life-threatening calls arrived within eight minutes, missing a target of 65 per cent.
At business questions in the Senedd, opposition AMs called for a statement from the Assembly Government on the future of the service.
Business Minister Jane Hutt said: "Let's make sure that our role is to ensure that the Welsh Ambulance Services Trust continues to improve with increased stability, which is what I believe they are doing."
Shadow health minister, Andrew RT Davies, said: "Alan Murray has internationally recognised qualities that the Welsh ambulance service has benefited from for several years.
"His successor faces significant challenges in tackling the many problems facing the service."
For the Liberal Democrats, Peter Black, said: "When he took up post, Alan Murray inherited a poorly performing ambulance trust that was lurching from crisis to crisis. He brought stability and leadership to that organisation and worked hard to improve its performance."
Trust chairman Stuart Fletcher thanked Mr Murray, saying: "There is still some way to go but we are now in a stronger position as we enter the next stages of the modernisation programme. I would like to wish Alan all the best in the future."