The presenter made the trip to Westminster Hall with her mum and said it was "utterly magnificent".
She tweeted: "Evening - along with my lovely mum and her very good friend, I have just experienced a moment in history - witnessing the Queen lying in state in Westminster Hall.
"At once majestic and peaceful."
Evening - along with my lovely mum and her very good friend, I have just experienced a moment in history - witnessing the Queen lying in state in Westminster Hall. At once majestic and peaceful. pic.twitter.com/f6hhdI96L6
— Susanna Reid (@susannareid100) September 15, 2022
She went on: "Once you enter Westminster Hall, it is worth every minute of The Queue.
"Quiet. Respectful. Historic."
Reid also kept her followers updated about what the queue was like and shared plenty of tips, including that people should wear their "comfiest shoes" and "take loo roll".
She tweeted: "If you are planning to queue here are our tips.
"We joined at 1.23pm near Butlers Wharf and entered Westminster Hall at 8.43pm - 7 hours 20 mins.
"Wear the comfiest shoes you own.
"Go with someone if you can, although everyone in the queue was friendly."
The TV star, 51, said in another post: "Don’t carry too much in a bag - water is freely available and there are lots of cafes along the route.
"Also plenty of toilets.
"I put my phone on low battery mode and it lasted the entire time."
Reid added: "Take loo roll.
"Carry wet wipes for hands - not sanitiser as it will be confiscated by security."
The Queen’s lying in state at the Palace of Westminster is open 24 hours a day until it closes at 6.30am on 19 September, the day of the late monarch's funeral.
Watch: Susanna Reid pays tribute to 'inspiring' Queen Elizabeth II on GMB