“ Mobile -first” has been the battle cry in the last couple of years for every company online, from startups to Facebook , as more and more people flock to apps and the mobile Web from smartphones instead of their computers. The idea is that if you don’t design your website or ad to work on mobile devices first, you’re an idiot.

But a top Google GOOG +0.23% ad executive says that’s no longer good enough, at least for advertisers, app makers, online publishers and, most of all, consumers. Speaking at the semi-private RampUp online ad conference today in Mountain View, Neal Mohan, Google’s vice president of display advertising, said the key now and going forward is to develop ads that work across multiple screens.

“If you’re just focusing on mobile, you’re solving yesterday’s problems,” Mohan said at the conference, put on by the ad data firm LiveRamp . Throughout the day, he said, consumers are flitting back and forth among many devices, from smartphones to desktops and laptops to tablets to TVs.

“90% of consumers start a task on one device and finish it on another,” he noted. “Consumers are way ahead of where advertisers and publishers are.”

A a result, he said, ad formats need to work on multiple devices simultaneously. “It needs to be something that can seamlessly take advantage of the characteristics of each device,” he said. “Our messages need to adapt to that. When I talk to brand and advertisers and agencies, I don’t talk about something that’s device-specific.”

A skeptic might wonder if Mohan’s insistence that multi-device rather than mobile-first advertising is self-serving, since Google has seen falling ad prices largely blamed on advertisers paying less for mobile ads. But it’s clear that people are growing increasingly comfortable juggling their online activities among many devices–with more, such as smart watches and Google Glass, still to come.

Google’s not alone, either. “In two years, we won’t be talking about mobile,” Greg Coleman, president of ad tech firm Criteo, said at conference panel. “We’ll just talk about reach.”

At the same time, this multiple-device behavior is affecting automated advertising variously known as programmatic or real-time bidding, in which ads are targeted to people in real time. ”That concept should apply regardless of the device,” Mohan said. “We’ve seen a lot of success with advertisers shifting from desktop programmatic to multiple screens. I’m just as bullish on programmatic selling on a multi-device world.”

Overall, Mohan said, the key to more advertising flowing from traditional channels such as television and print is not better technology but better ads. “All of this technology doesn’t matter unless the message is something that resonates with consumers. There’s no way of getting around that,” he said.

LiveRamp CEO Auren Hoffman and Google Display VP Neal Mohan

One way Google is forcing that issue is offering people the option not to watch an ad on YouTube or its display-ad network of some 2 million websites. TrueView ads on YouTube and Engagement Ads on the Google Display Network let people skip an ad or opt to see a bigger one, and while many skip ads, many do not. “We see a 3% engagement rate [on Engagement Ads], compared with a 0.1% click-through rate” of most display ads. That’s the way most advertising will happen in the digital world.