She takes over from Sir Alistair Graham whose three-year term came to an end on 25 April.
The committee is a non-departmental public body set up to examine concerns about the standards of conduct of all holders of public office.
Donaghy, who is the current chair of conciliation service Acas, has been appointed to 31 December 2007. She has agreed to step down should a permanent chair be recruited in the coming months.
She has been a member of the committee since March 2001 and is also an ex-president of the TUC.