Inverness were relegated from the Ladbrokes Premiership despite a 3-2 win against Motherwell.
Hamilton's 4-0 victory over Dundee rendered the Caley Jags' result redundant, condemning them to the Championship for the first time in eight years.
Inverness's goals came in a frantic four-minute period, with Greg Tansey and an Alex Fisher brace giving the Highlanders a comfortable advantage. James McFadden pulled one back for the visitors before Ryan Bowman's stoppage-time penalty.
Inverness held on to win but many difficult decisions will need to be made this summer, such as what happens with manager Richie Foran and the playing staff as well as potential changes in the boardroom.
Given they had reduced a seven-point deficit a fortnight ago to just one before kick-off, many had believed a great escape was now possible after Caley looked doomed for so long.
A miserable day in the Highland capital failed to put off a bumper home crowd, who ignored the downpour in Inverness to give their team a rousing backing.
But they must now prepare for trips to Dumbarton and Queen of the South next season, likely watching a much-changed team.
Caley Thistle named the same XI that beat Dundee 2-0 in midweek, while Motherwell replaced David Ferguson, Scott McDonald and Keith Lasley with Lionel Ainsworth, Shea Gordon and Allan Campbell.
The task for Inverness was simple - win and hope Dundee did them a favour against Hamilton. They had won two of their last three and Accies had lost their last four, which meant an escape was not impossible.
A driven effort from Louis Laing was the early sighter Caley Thistle needed to settle any nerves, with goalkeeper Russell Griffiths getting down to parry wide.
Ainsworth had Motherwell's first chance, slaloming through the Inverness defence and rolling a shot across the face of goal which Ryan Esson watched nervously.
Liam Polworth was the main goal threat early on and he turned on a smart ball from Iain Vigurs and headed for goal, only to send his left-footed effort over.
The bad news for Inverness was that before they broke the deadlock, Accies were two goals up against Dundee. Chris Cadden's run and shot, taking three Caley Jags out of the game in the process, did little to improve the mood.
Inverness started the second half with little of the verve needed to make a great escape happen, with Gary Warren coming the closest.
But Tansey sparked the goal blitz on 64 minutes, walloping in from Polworth's lay-off, then two bits of opportunism from Fisher - both from Ross Draper miscues - put the Caley Jags three clear.
McFadden scored with his first touch to scupper any chances of a clean sheet, but the fact Accies were drubbing Dundee meant few cared.
The closing stages were a non-event until David Raven's foul on Craig Clay Bowman's penalty conversion, ending Caley Thistle's seven-year stay in the top flight.
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