Mulcair smiled, walked up to Stornoway's front step and unlocked the door as cameras clicked.
"It's a real honour," Mulcair said before entering.
It's been a while since the mansion, built in 1913, was home to anyone. The late Jack Layton stayed just one night at Stornoway last year, spending much of his time under medical care in Toronto. Interim leader Nycole Turmel never moved in while at the party's helm.
Mulcair won the NDP leadership March 24, and first set up shop in the Opposition leader's office on Parliament Hill, hiring staff and working on his shadow cabinet, expected to be announced later this week.
Stornoway is located less than two kilometres from the prime minister's official residence at 24 Sussex Drive. Canada's Opposition leaders have lived in the 34-room mansion since 1950.
Layton held a similar photo-op when he moved into the residence last year. He said one word as he stepped inside the building: "History."