Revealing he was feeling "really emotional", Ant said: "I've missed you all".
McPartlin stepped away from presenting roles on Britain's Got Talent, I'm A Celebrity and Saturday Night Takeaway in an attempt to try and tackle his demons.
Turning to Dec, he said: "As for you, I've missed you".
The auditions are taking place at the London Palladium on Friday afternoon, with judges Simon Cowell, Alesha Dixon, Amanda Holden and David Walliams all expected to attend. Thank you very much, what a warm welcome.
The audience cheered as the presenters hugged. "Shall we move on before I cry?"
While the audition process for Britain's Got Talent will begin this month, the show is not due to air on ITV until springtime this year.
Earlier in the day, McPartlin and Donnelly had arrived together at the back entrance of the London venue in a black SUV, both waving and smiling at fans as they got out of the auto.
"Ant welcome back. We missed you, it wasn't the same".
One person took to Twitter to say, "Nice to have you back Ant. Dec is great and all that but we all know you're the amusing one". I mean, even though Dec was brilliant, it still wasn't the same.
It was the first time the infamous duo had been together for work since Ant took a break from the limelight following his drink-driving arrest past year.
He was subsequently fined £86,000 and given a 20 month ban on driving after pleading guilty to drink-driving. "Can't wait to see you back on TV soon", posted a second.
Britain's Got Talent returns to screens in the Spring.
McPartlin, who divorced wife Lisa last year, previously struggled with a two-year addiction to super-strength painkillers following a knee operation.
He was twice over the legal limit when he was involved in a collision with two other cars in Richmond, South West London on 18 March.