The One Direction heartthrob first went to Ariana's show, part of her 'The Honeymoon Tour', and then joined the 'Love Me Harder' singer backstage.
The pop pair were then later seen hanging out at Soho night spot Cirque De Soir until the early hours of Tuesday morning, but each travelled to the club on their own and left separately.
Website Mirror.co.uk reported their evening out was brought to a sudden end after an electrical fault at the club resulted in three fire engines racing to the venue.
Earlier in the evening, Ariana was excited to get to meet 'Harry Potter' stars Matthew Lewis and Evanna Lynch backstage at The O2 and introduced the pair to her pet dog Sirius - who is named after 'Harry Potter' character Sirius Black, who is able to take on the form of a canine.
Massive Potter fan Ariana took to Instagram to post a selfie of them all having a magical time and the photo of the pop princess with the two actors - who played characters Neville Longbottom and Luna Lovegood - was accompanied by a caption which read: "Neville, luna and sirius all in one photo. @msevylynch told me i'm the sweetest Slytherin she's ever met. thank you @msevylynch @realmattdavelewis for coming to my show and making my night!!! i adore y'all. #TheyWalkedInAndDeathlyHallowsWasPlayingOnMyTv #MyWifiPasswordInMyDressingRoomIsHarryPotter #iwasembarrassed #ohwell (sic)"
This is the latest of several nights out for the 1D star, who was recently seen in the company of Little Mix's Jade Thirlwall on Saturday night (30.05.15) and a member of pop group Fifth Harmony on Sunday (31.05.15).