Dorset Healthcare foundation trust reached double excellent with an improvement in its resources rating from good in 2006-07.
Roger Browning, chief executive since 1992, says it has benefited from management consistency while encouraging change.
An excellent resources score was a particular success for a low income mental health trust that had to carefully manage its costs, says Mr Browning.
It has broken even for the past 16 years, and plans to invest£35m in improvements over the next three years.
Mr Browning says: "Our vision statement is not to be the best in standards, but to provide services which any of us would be happy with and would recommend to our family or friends."
Most board members have been in position for at least five years, the chief executive says, which helps with continuity.
But he adds: "I am aware we can actually do better and a message we do give to staff is that we are not here to keep things the same."