Some Republicans assumed that Staten Island Assemblywoman Nicole Malliotakis wasn’t serious when she announced in April she was considering a run for mayor.

“There was the whole issue whether she was just raising her profile. People didn’t know if she was in, or she would drop out,” said Manhattan Republican Party Chairwoman Adele Malpass.

Two months later, the daughter of Greek and Cuban immigrants is the GOP’s presumptive nominee to take on Democratic incumbent Bill de Blasio after her chief primary rival, businessman Paul Massey, quit the race.

And in a short period of time, Malliotakis has proven she’s the real deal, Malpass said.

Malliotakis, 36, immediately filed papers declaring her candidacy, participated in forums, stumped across the city and won the additional backing of the Conservative Party.

“The [Republican] party is going to unite behind her. Over the last two months she has proven to be an energetic, knowledgeable campaigner who could make Bill de Blasio a one-term mayor,” Malpass said.

Malliotakis — who would be the city’s first female mayor — is familiar with being an underdog.

Her initial claim to fame is defeating former Democratic Assemblywoman Janele Hyer-Spencer — a close pal of former Assembly Speaker Sheldon Silver — in 2010 in a district that includes parts of Staten Island and southern Brooklyn.

“I was outspent 2¹/₂-to-1 against Shelly Silver’s favorite member. I had no union support,” Malliotakis recalled.

“I defeated an incumbent once. We can do it again.”

Speaking of her Cuban mom and Greek dad who came to America without a command of the English language, she said, “This exemplifies how special America and New York City are. In one generation their daughter is running for mayor.” Her parents became American citizens in 1971.

The assemblywoman is happily single. “It’s difficult to find someone who wants to put up with my schedule,” she said.

Her favorite historical political figure is President Theodore Roosevelt, who also served as New York governor at the turn of the 20th century.

“Teddy Roosevelt was a reformer who always did what was right, regardless of party,” she said.

Malliotakis long ago was bitten by the ­political bug.

She was senior class president at New Dorp High School, from which she graduated in 1998.

She served as an aide to former Gov. George Pataki, working on New York City issues, and also worked as a staffer to late Staten Island state Sen. John Marchi, who twice ran for mayor.

A graduate of Seton Hall University and Wagner College, Malliotakis also was employed for four years as a public affairs manager for Con Edison before her election to the Assembly.

A fitness buff, Malliotakis is a yoga enthusiast who sponsors classes for the public in Midland Beach and Bay Ridge. She also bikes and has taken boxing-fitness classes in Albany.

As for the campaign ahead, Malliotakis promises to be a take-charge mayor — especially when it comes to fixing the transit mess.

“I would be on the phone today reaching out to the governor and [to MTA chief] Joe Lhota making a financial commitment to the subway system,” she said.

She hit de Blasio on the homeless crisis, the failure to stop child abuse and for going soft on illegal immigrants who commit crimes.

She even used de Blasio’s “tale of two cities” theme to tweak him — when it comes to the “unfair” property tax system.

“The value of my home is a little more than a third of Bill de Blasio’s, yet I’m paying more in property taxes,” said Malliotakis, vowing an overhaul.

During her debate against Massey before his withdrawal Wednesday, she showed her sense of humor.

Asked what her campaign slogan might be, she responded: “Get New York City out of the hole, so vote for Nicole,” eliciting laughs.