It was the most memorable birthday song since Marilyn Monroe serenaded John Kennedy.

On Sunday’s season premiere of “Mad Men,” Don Draper’s new wife, Megan, decided to give her man a surprise 40th birthday present, one that he and his colleagues in attendance wouldn’t soon forget. “You’ve never seen me throw a party,” Megan told Peggy. “Everyone’s going to go home and have sex.”

Dressed in a slinky black dress with flowing sleeves and a hemline that left little to the imagination, Megan sat Draper in a chair, grabbed a microphone and cooed a coquettish version of “Zou Bisou Bisou,” complete with sexy hip shimmies and a big finale that ended up with the leggy lady perched on her husband’s lap.

The effect on Don was mixed. The effect on the audience more immediate. Yesterday morning, pretty much every “Mad Men” viewer had one question on the mind: Who was that girl?

That girl is Jessica Paré, a Canadian-born actress who had a supporting role last season but, after last night’s episode, seems poised to become one of the show’s regulars. Whereas Joan used to keep the office entranced with her curvy figure, while Peggy wowed everyone with her brains, Megan might possibly be a combination of the two: sexy and smart.

“It’s surreal,” Paré tells The Post of her new stardom. “I went from not knowing whether I had a job or not to having all these incredible scenes. It’s like a dream come true.”

And oddly enough, that dream started with a single line, “Yes, Joan.” Paré’s character appeared in the second episode of last season as a secretary. By the final episode, Draper was proposing to her.

Paré, 31, says most of her character’s plot developments are a complete surprise to her because the show’s creator, Matthew Weiner, is notoriously secretive. The first time she had an inkling she was to marry Draper was when a wardrobe person fitted her for a ring. She also knew next to nothing about this season’s premiere.

“Matt did explain to me that Megan had a song-and-dance number, ” Paré says. “[These] are not my fortés, so I feel less comfortable doing those things,” she says. “It was a little embarrassing.” Limited ability or not, that is her singing voice on the episode. She recorded the track in a studio in California, and it’s now available on iTunes.

She had another breakout scene toward the end of Sunday’s episode when her character attempts to torture Draper by cleaning their apartment wearing nothing but black underwear. “It’s a fantastic piece of writing, so I was nervous as an actress, but I wasn’t nervous to wear underwear,” she says. “[It] had a lot more coverage than a lot of bikinis I wear.”

In typical “Mad Men” style, even Paré’s original audition was cloaked in mystery. She tried out for another character, but was invited back two more times for the Megan role, which at the time didn’t have a name attached to it, just a description: female brunette. (The last name, Calvet, was ultimately inspired by a Montreal B&B Weiner stayed in.) “As I left the room [after the audition], they thanked me for coming in. So I thought for sure that I didn’t get it.”

A week or so later, she got the call. “I went out for lunch, which was previously a luxury that was not afforded,” she says.

The bilingual actress was born in Montreal, the daughter of a professor and an interpreter. She says she first got bitten by the acting bug at age 12, when she helped her father, who was an actor, learn lines for his part in a production of “The Tempest.” She landed her first paid gig at the age of 17 in the 1999 TV movie “Bonanno: A Godfather’s Story.”

She moved to Los Angeles eight years ago and secured smaller roles in “Hot Tub Time Machine” and Canadian films, including “The Trotsky,” but she struggled to break out. “It’s a tough business,” she says. “I was pretty close to throwing in the towel. Then ‘Mad Men’ came along.”

Her next stop could be Hollywood. “I have not yet received any offers, but I’m waiting for them to roll in,” she says. “I’m open to all opportunities, but everyone has the dream of doing a big Hollywood blockbuster, for sure.”

For now, she’s content to work on the series, while painting, knitting, reading and playing the ukulele (badly) in her spare time. After splitting from her producer husband, she now has a boyfriend whom she declines to name. As for the future of Megan on “Mad Men”? “There will be a next season, but I don’t know if I’ll be in it yet,” Paré says. If not, expect to see casting agents giving her roles elsewhere; they’d be mad not to.