TV

New ‘Tonight Show’ gig serious business for Fallon

Taking over the “Tonight Show’’ is no laughing matter for Jimmy Fallon.

The new host — who debuts Monday night — looked tense and preoccupied as he walked his dog outside his Gramercy Park pad Sunday.

Asked if he’d be doing one of his favorite bits, “Slow-Jam The News’’ — in which he sets dry current events to a groovy Barry White-esque beat — on his new show, the uncharacteristically stoic star said only, “We’ll do something fun.”

Among Fallon’s first guests Monday will be movie star Will Smith and U2.

The “Saturday Night Live’’ alum will host the show from NBC’s Studio 6B at 30 Rockefeller Center.

In taking his seat behind the “Tonight Show” desk, Fallon will be occupying a comedic throne previously held by legends such as Johnny Carson, Jack Paar and Steve Allen.

He moved the show back to New York roots 42 years after Johnny Carson took it to Burbank, Calif.

The show will premiere Monday with a later-than-usual midnight start, thanks to the network’s Olympics coverage.

It will go to its regular 11:34 p.m. start time Tuesday.

“Tomorrow is the day!!!’’ Fallon tweeted earlier Sunday.

The “Tonight Show” Twitter page also hyped the event by posting a picture of Fallon on Sunday holding a sign that read: “Tonight Show Countdown: 1 Day!”

The pressure will be on Fallon to keep up the solid No. 1 late-night ranking left to him by his ratings-snaring predecessor, Jay Leno.

Additional reporting by ­Gregory P. Mango