It gives ABC another talking point about how it is adapting to audience preferences; in this case, viewers will be able to carry “Good Morning America” with them as they move around the house in the morning, or tune into a weekend basketball game while out with friends. The live stream will work anywhere in a local market, the same way an old-fashioned TV antenna would.

During a demonstration of the app in her New York office on Friday, Ms. Sweeney said she was struck by how personalized television becomes when it is live-streamed to a person’s phone.

The app is also an implicit rebuttal to Aereo, the start-up backed by Barry Diller that is being sued by major station owners for streaming their signals to paying subscribers in New York. Ms. Sweeney reiterated her view that Aereo is illegal but said the plans for the app’s live-stream feature predated the service.

The app, to be named Watch ABC, in line with Disney’s existing Watch Disney and Watch ESPN apps, will allow users to watch ABC shows on demand, like the network’s previous app had. In the future, ABC will withhold its most recent TV episodes from the free versions of Hulu and ABC.com, further limiting access to paying subscribers of cable and satellite providers only.

The mobile live stream will not carry the same ads as the television broadcast; instead, it will include the same sorts of digital ads as on ABC.com. This is in part because the Nielsen Company is not able to measure mobile viewing of live television yet.

“What you see here is the same live programming,” Mr. Cheng said as he used the app, “but what we are doing during the commercial break is actually inserting new ads into the stream.”

Over time, live-streaming of ABC stations could cannibalize big-screen viewing of those stations, but ABC could make up the difference through streaming ads. Disney’s chief executive, Robert A. Iger, pointed out this month that an increase in online advertising partly compensated for declines in TV ad revenue in the first quarter of the year.