Mr Reid has held some of the most high-profile government posts
John Reid was always going to keep Gordon Brown guessing about his leadership intentions until the very end.
The surprise is that having decided not to stand, he is going to quit frontline politics and return to the backbenches.
Clearly, as a true Blairite believer, he either wanted to carry the torch of reform himself as Labour leader or else quit the scene altogether to make way for new blood.
Unlike many of his Cabinet colleagues, Mr Reid had little enthusiasm for serving under Mr Brown, recently joking he would get the job of the tea boy in a Brown government.
After the drubbing Labour received in Thursday's elections, he detected little appetite in the party for a potentially divisive leadership contest - and, no doubt, calculated he would not get enough support to mount an effective challenge.
Besides, as he points out in his resignation statement, after nine ministerial jobs in 10 years, he could probably do with a break.
A tough-talking reformed drinker, once jokingly described as a "safe pair of fists", Mr Reid quit the bottle in the mid 1990s and dedicated himself to spreading the New Labour message.
After proving himself in a string of junior ministerial roles, he became Tony Blair's favourite firefighter - ever ready to be parachuted into troubled departments to knock heads together.
He took on the crisis-hit Home Office brief last year with some reluctance, saying with typical loyalty "when Tony Blair asks me to do something I do it".
He immediately grabbed headlines - and ruffled civil service feathers - by declaring parts of the organisation "not fit for purpose".
Since then, he has announced that the department will be split into two as it seeks to rebound from a string of high-profile errors.
The son of a postman, he gained a PhD in economic history.
He threw himself into the tempestuous student politics of the time and was briefly a member of the Communist Party.
He worked as a research officer for the Scottish Labour Party and then as adviser to Labour leader Neil Kinnock from 1983 until 1985.
He entered the cabinet in 1999 as Scottish secretary but his career was nearly ended when his lobbyist son Kevin was taped telling an undercover reporter: "I know the secretary of state very, very well, because he's my father."
He was then reprimanded by parliamentary standards commissioner, Elizabeth Filkin, over claims he had used taxpayers' money to employ political researchers, including his son.
But Ms Filkin's findings were rejected by the committee of MPs to whom she reported.
Mr Reid went on to become the first Roman Catholic to be Northern Ireland secretary - replacing the sacked Peter Mandelson - where he oversaw the IRA's first act of decommissioning.
Mr Reid was seen on TV when an alleged terror plot was foiled
Next stop on the ministerial merry-go-round was a stint as Labour chairman before becoming leader of the House of Commons.
Then, in his third ministerial post in a year, he became health secretary, where he had to tackle the thorny issue of calls for a ban on smoking in enclosed public places.
He drew up plans for smoking to be banned in pubs and restaurants serving food but not in other pubs and clubs.
The proposals were later branded "unworkable" by MPs and the government eventually gave a free vote in which Mr Reid's successor, Patricia Hewitt, voted for a full ban.
Mr Reid left the health job in May 2005 to become defence secretary, the post he had coveted since serving as armed forces minister during Labour's early years in power.
During his year in the job, he took the decision to send 3,700 extra British troops to southern Afghanistan to help reconstruction efforts and fight the Taleban.
He finally moved into one of the top positions in government when Charles Clarke was sacked as home secretary in May 2006.
A passionate advocate of Blairite public service reforms, he was regularly touted as a potential "stop Gordon" leadership candidate.
And - as one-by-one his Cabinet colleagues fell in line behind Mr Brown - he pointedly refused to rule out a challenge, making a series of provocative speeches on Labour's future.
He relationship with Mr Brown is reported to be frosty, although the precise source of their dislike has been lost in the mists of time.
Former Labour colleague George Galloway has said at one time they were "as thick as thieves".
They cut their teeth together in the rough and ready world of Scottish Labour politics in the 1970s and 1980s, but friends from the era say the then hard-drinking Mr Reid was seen as potentially unreliable by the more cautious Brown.
He hit a low point in the early 1990s, when he famously threw a drunken punch in the Commons chamber as he tried to force his way past an attendant for a vote.
According to Labour folklore, he quit drinking within 24 hours of being told to do so by then leader John Smith.
In 1998, he was struck by personal tragedy when his wife Cathy died suddenly from a heart attack.
The couple met at a school party as teenagers and went on to have two sons.
Two years after the death of his wife he started a relationship with his present partner, film-maker Karine Addler. He is reported to have given up his 60-a-day cigarette habit in the hope of having more time with her.
Mr Reid lists his hobbies as football, history, crosswords and playing the guitar.