Harbour was nominated for an award at the ceremony and before the bash officially kicked off, the couple hit the red carpet together.
Singer Allen, 34, sported a stunning red and pink gown with a plunging neckline for the occasion.
The star wore her hair slicked back and accessorised with simple, diamond drop earrings.
Harbour, 44, was suitably dashing in a suit and bow tie.
The two stars began dating last summer after being spotted on a date in London.
After a handful of other outings in the capital, Allen flew over to New York where she helped Harbour celebrate his Saturday Night Live hosting debut.
They also attended a basketball game at Madison Square Gardens.
The Stranger Things star was up for the Best Actor in a Drama Series accolade but lost out to Game Of Thrones actor Peter Dinklage.
The Stranger Things cast was also on the shortlist for the Outstanding Performance by an Ensemble in a Drama Series award, which was won by their Netflix stablemate The Crown.
Other winners on the night included Jennifer Aniston, who was recognised for her work in the new Apple TV programme The Morning Show.
See the full list of winners here.