The One Direction star joined the headliner for their 2017 duet "Seeing Blind", and Maren was so pleased her friend could join her at the prestigious venue, she went back to her hotel and sobbed after the show, which wrapped up the international leg of her "Girl: The World Tour".
"Headlining that room has been a goal of mine for so long and last night, it felt like a lucid dream," she wrote on Instagram on Saturday. "I cried when I got back to the hotel because it's bittersweet that this tour is done, but holy s**t, the memories that have been made with my amazing band, crew, and you guys."
The big gig wasn't lost on Niall, who posted a photo of himself and his "buddy" on stage at the Royal Albert Hall before fans arrived and wrote: "Great to catch up with my buddie (sic) @marenmorris and jump on stage with her at @royalalberthall last night. What an amazing show!".
The gig came just days after Maren was joined onstage by another special guy in her life - husband Ryan Hurd.
The fellow country singer surprised Morris with a visit before her show in Dublin, Ireland last month, and stuck around to join her onstage in Leeds, England last Sunday (May 27).
Maren will now head back to the U.S. and resume her tour there.
The "My Church" singer previously revealed her 2018 summer tour with Horan was a big test for her marriage.
"We have grown so much closer because we had to make some hard decisions and have some tough talks about what this future looks like and how we can make this better," she told Esquire. "I have always loved him, but I feel like I like him in ways I never knew I could before."