The Dynasty actor reached the milestone this month and marked the occasion with a party in London.
She also celebrated with a special dinner, and Morgan shared a picture from the evening on Instagram.
"The birthday girl!" wrote the broadcaster.
"Lovely dinner to celebrate the great Dame @joancollinsdbe, who as usual looked younger than me, and infinitely more glamorous."
Fans agreed, with many saying Collins looked more youthful than the 58-year-old TV star.
"She looks fabulous and I agree with you but she does look younger," one person wrote in the comments section.
"What an amazing woman!" said another.
"And yes she does look much more glamorous AND younger than yourself, must’ve had a hard paper round."
"Aged perfectly and still glam," posted one fan.
"Wow she’s never changing," remarked another.
Somebody else said: "She is such an inspiration for someone who has never given up and keeps the spirit of youth. She is an icon!"
"She never ages!" exclaimed another admirer. "Such an incredibly lovely lady."
Somebody else chimed in: "Wow how can Joan be 90. She looks amazing. You don’t look too bad either Piers x."
Actor said how you 'feel' is more important than age
Last year, Collins - best known for playing catty Alexis in the US show - said in an interview that in her opinion age was not everything.
"But you know, people have been calling me an older woman since I was 38," she told Saga magazine.
"I think what is important is not age, but how you look, feel and behave."
Watch: Dame Joan Collins attempted to invite Princess Diana to dinner but told "go away" by security guards