Cantwell thanked viewers as he bid adieu to the Ballymount station where he has worked since its launch in 1998.
"I've enjoyed bringing you the news for the past 14 years," he said at the end of the 5.30 show. "Thanks very much."
Alan said it was an emotional day for him, but he was looking forward to the future.
"It's a sad day, but it's an exciting day. It's time to draw a line in the sand, leave the past behind and look forward to the future."
Alan and his co-workers celebrated his time at the station with a glass of bubbly after his final broadcast.
But the former news anchor said he wouldn't be going too wild on his last night. "I'll have one or two drinks, but I'm not a big party guy."
Alan's decision to leave surprised both TV3 bosses and viewers.
But the 45-year-old said he felt it was something he had to do. "The team here weren't that enthusiastic about the decision, but I needed to go.
"I reached 45 and at the beginning of last year decided I could not be at TV3 in 2013."
His wife Denise encouraged him to take the leap: "She was incredibly supportive. She could see that I was dragging my feet and she pushed me over the line. I needed that."
And while Alan would consider moving to the States, nothing is set in stone.
"I have a lot of balls in the air and I don't want to talk about any of them till one lands in my lap."
But he is adamant that he will remain in broadcasting: "It's what I know and it's what I'm good at."
Co-presenter and Herald columnist Colette Fitzgerald will take over and he says she's perfect for the job.
"A woman of Colette's calibre is capable of carrying not just the news on her own; she is capable of carrying the entire station on her own."