If McCann hoped its star turn on the final season of "Mad Men" as Sterling Cooper & Partners' majority owner would offer some fun moments of nostalgia and publicity, the show's writers haven't made it easy.

Portrayed during the midseason return as a bureaucracy staffed by dumb sexists, the kind of place where Joan leaves a meeting wanting to "burn this place down," the McCann of "Mad Men" has now hardened into an assertive villain. When a junior McCann exec insults Joan's client in the May 3 episode "Lost Horizon," a senior executive offers to help -- via a road trip for two that Joan doesn't want. "Hey, I'm easy," he says. "I'm not expecting anything more than a good time."

She's eventually bought out for far less than her stake is worth.

Harris Diamond, chairman-CEO of the real-life modern-day McCann, said the agency was caught a bit off guard by what has happened in the final season. "It was both fun and a little bit surprising," he said.

Neither AMC nor "Mad Men" writers asked McCann for input, according to the agency. "In fairness, I don't think they were looking for permission," Mr. Diamond said.

But the agency is taking it all in stride. Given how the previous half-season left off, McCann made ID cards for Don, Peggy, Roger, Joan and Pete ahead of the show's return, along with a welcome sign in the agency's common area. But without advance knowledge of the final arc, the agency's response since then has come in real time.

As McCann's (fictional) sexist reps completely dismissed Joan and Peggy in the midseason premiere, for example, the agency posted on Twitter: "This meeting is so awkward. Our apologies on a purely theoretical basis. #MadMen"

As the season progressed, the shop posted a photo of Darth Vader and joked, "Hey guys. It's not so bad here! #MadMen."

After "Lost Horizon," the agency answered a Twitter inquiry from a Daily Beast reporter by saying, "Hi @eshire. What do you want to know? We're kinda busy subjugating our new staff."

The experience echoes Jaguar's bumpy ride on "Mad Men" in 2012, when writers had a Jaguar dealer offer to support Sterling Cooper Draper Pryce in a pitch in exchange for one night alone with Joan. "I'm a big fan of the show and it was gratifying to see our brand portrayed," David Pryor, VP-brand development for Jaguar USA, told Ad Age then.

"I would say we were fairly surprised at the turn of events," he added, laughing.

Despite the fact that Mad Men is a fictional series set more than 40 years ago, Mr. Diamond said the ad business and society still have work to do. "We all know this industry has had issues," he said. "We know we still need to be more diverse. It's not funny, and frankly I think we can still do better. And this goes beyond just advertising -- we all have to do better."