Gary Holt's precious 73rd-minute goal removed the need for Kilmarnock to enter a play-off with Airdrie to see who would be in the Premier league next season.
That dubious honour now goes to Hibernian who will face their First Division opponents over two legs on 18 and 25 May, while Kilmarnock can concentrate on their Scottish Cup final appearance. Robson must wish Middlesbrough could be so equally assured before Wembley.
The anticipation of the final helped that special festival feeling descend on Rugby Park, as long queues snaked around the car park for both tickets and souvenirs that no self-respecting fan would be without for the big day out at Ibrox week Saturday.
However, the mood is not something that Kilmarnock's players have been able to share in. The twists of the tail-end of the season have kept their future membership of the Premier League in doubt until the last.
With Motherwell and Hibs both joining Williamson's team in entering the final day seeking the victory that would hand the loathed play-off place to anyone other than themselves, it was always certain to be a day of tension - and ears welded to radios for news from elsewhere.
Kilmarnock were hindered by the loss of top-scorer Paul Wright, badly injured last week, and his partner Jim McIntyre needed a late fitness test. He was shown little sympathy by Aberdeen, whose full-back Russell Anderson was booked after just six minutes for a scything tackle on the striker. Kilmarnock's own right back, Gus McPherson, created the first real goal threat in the 17th minute, thundering a 20-yard shot which flew just inches wide of the posts.
However, Kilmarnock were indebted to their goalkeeper, Dragoje Lekovic, for protecting the parity in the 26th minute. The Yugoslav international produced a breathtaking reflex save to block a bullet header from Billy Dodds. The striker did not have to wait long for his 24th goal of the season as seven minutes later he put Aberdeen ahead and sent Rugby Park into a deathly silence, rising to head in Joe Miller's perfectly flighted cross. McIntyre, meanwhile, had been booked for evening the score on his early assailant Anderson and news of goals before the interval for both Hibs and Motherwell were further signs that things were not going to plan.
The visitors' intentions to close down the game became clearer with the introduction of Jamie Buchan - son of former Manchester United captain Martin - as a substitute to give them five at the back.
However, Kilmarnock finally drew level in the 73rd minute when Holt was in a perfect position to head past the Aberdeen keeper, Derek Stillie, to finish off a lovely piece of skill by McIntyre.Reuse content