Ms McDonagh, 38, succeeded the veteran trade unionist Tom Sawyer when she took over the top post in October 1998. Although she is a close supporter of the Prime Minister, she has been dogged by criticism.
The youngest occupant of the post, and the first woman, her elevation owed much to the strong impression she had made on the Labour leader during her tenure in the London Region. She was seconded from the London office to party headquarters at Millbank to help out with the general election campaign, and proved to be a highly competent organiser.
Ms McDonagh rose to become deputy general secretary, and was clearly being groomed for the top job, but some opponents attempted to scupper the move at the last minute.
Anji Hunter, Mr Blair's special assistant, helped to deliver a majority of the important National Executive Committee votes for Ms McDonagh, despite the uncertainty of some members worried about her youth and relative lack of experience.
Yet after her coronation at the 1998 party conference, the doubts and charges of "control freakery" grew as Labour became embroiled in the contest for Welsh First Minister between Alun Michael and Rhodri Morgan.
Her critics claimed that Ms McDonagh, who lives with her sister, Siobhan McDonagh MP, was too much of a "Blairite clone" and complained that she failed to carry out the traditional role of telling the leader exactly how the party in the country felt.
Her hard-edged style, coupled with her unashamed appointments of Blairite regional officers to Millbank posts, have also led to some key staff leaving the Labour headquarters.
However, another of the possible scapegoats for the fiasco over the mayoralty is Sally Morgan, Mr Blair's political secretary and his vital link with MPs and with the wider party.
Ms Morgan is understood to have had a fraught relationship with Ms McDonagh ever since she was first touted as a contender for the general secretary post.
This important breakdown in relations between Millbank and Downing Street is said by senior insiders to lie at the heart of the chaos over the mayoral selection.
Ms McDonagh repeatedly tried to make Mr Dobson stand for the post, but he initially refused. As health secretary, and with little to gain from standing down, it is understood that he was unimpressed by Ms McDonagh's style. In September, Downing Street became convinced that Nick Raynsford had the all-clear to run for the post of London mayor, even as Mr Dobson was in the process of preparing his own candidacy. Wires between Mr Blair's two key advisers became seriously crossed.
However, Ms McDonagh's supporters point out that her hands were tied over the issue, both by Mr Blair's refusal to act urgently and by the unwillingness of both Mr Dobson and Mo Mowlam to enter the mayoral race.Reuse content