Yesterday, not a penny richer, he returned to California after resigning as chief executive on his first day in the job. His departure, on medical grounds, plunged the bank into turmoil, leaving it rudderless for the second time in five months. Mr O'Neill, 52, a former US marine with a glittering career in American banking, was hailed as Barclays' great hope when he was appointed in February, after the surprise resignation of his predecessor, Martin Taylor, last November.
But a black cloud already hung over Mr O'Neill when he flew to Heathrow on Sunday. Six days earlier, he had been to see his doctor in San Francisco because of two black-outs he had suffered after a bout of severe flu.
Last Tuesday, he telephoned Sir Peter Middleton, Barclays' acting chief executive. He told him that he had been diagnosed with an arrhythmic heartbeat - a serious condition that is aggravated by stress - and advised not to take up the post.
Sir Peter persuaded Mr O'Neill to come to London, where he was whisked off to Harley Street first thing on Monday morning for an examination by the bank's own doctor, who confirmed the American diagnosis.
After a meeting at Barclays' headquarters in Lombard Street his resignation was rubber-stamped at an emergency board meeting on Monday and announced to a stunned City at 7.30 yesterday morning.
Mr O'Neill - poached from the Bank of America, where he was chief financial officer - had already delayed his starting date by three weeks because of the flu.
He had been to London several times since his appointment and had been house-hunting in Hampstead, where he has owned a home in the past. His wife was planning to join him on Thursday, and the rest of the family - two sons and two red setter dogs - were scheduled to move to London in the summer, at the end of the school year.
Mr O'Neill had also already bought pounds 5m of Barclays shares with his own money, a condition of his receiving another pounds 5m stake as part of his pay package.
But, having resigned before he reached his desk, that fabled package - the envy of the City - never materialised. Sir Peter said yesterday that Mr O'Neill had not received a pay-off. "He has taken nothing - neither in terms of salary or shares."
He is to be reimbursed his travel expenses: a clutch of return flights from San Fransisco to London and a few hotel bills. It was not for the money, however, that Mr O'Neill took the Barclays job; he already has a personal fortune estimated at $50m, and had been relishing a challenge that would have crowned his career.
Before flying home yesterday, Mr O'Neill said he was "deeply disappointed" to give up "a dream appointment" that he described as "the culmination of my whole career".
Colleagues put it more strongly; he was "devastated", said one, his life had been "completely flipped around".
Barclays said it was "disappointed", which was another understatement. Mr O'Neill was paraded as a great catch by the bank, after Mr Taylor resigned in response to a series of boardroom disputes, and Barclays is left again without a chief executive at a time of great upheaval in the industry.
A spokesman for the bank said events could not have been foreseen. "Mike had an intensive three-hour medical earlier this year which he passed with flying colours ..." he said. "He is 52 years old and seemed to be fit and healthy. The flu brought out or aggravated something that could not have been detected in the medical, and clearly it would not have been right for him to take up this job."
There was one small consolation yesterday for Mr O'Neill, who has decided never to work again. The price of Barclays shares has risen by pounds 2 since he bought his stake in February, thus earning him pounds 600,000.
Merger talks, page 15
Business Outlook, page 17
How pounds 10m Job of a Lifetime was Won ... and Lost
27 November 1998: Barclays' chief executive, Martin Taylor, surprises City by announcing resignation.
11 February 1999: Barclays unveils Taylor's replacement: former US marine Michael O'Neill, headhunted from Bank of America.
12 February: O'Neill's pay package revealed to be pounds 10m over three years, comprising pounds 1.7m salary including perks, guaranteed pounds 850,000 first-year bonus, pounds 5m of Barclays shares and lucrative pension rights.
22 March: O'Neill's scheduled start date at Barclays. Postponed by Mr O'Neill because of illness, said to be flu.
5 April: O'Neill sees doctors in San Francisco after suffering two black-outs during his illness. Diagnosed with arrhythmic heartbeat, a serious condition that is aggravated by stress.
6 April: O'Neill telephones Sir Peter Middleton, Barclays' acting chief executive, to say that doctors have advised him not to take up post.
11 April: O'Neill arrives at Heathrow.
12 April: Barclays conducts medical and confirms diagnosis. Emergency board meeting accepts his resignation.
13 April: Bank confirms his departure at 7.30am.