Three years later, the president of Gold and Stock Telegraph Company woke up to a burglar in his home, which inspired him to create a telegraph-based alert system.

With the increase in telephone usage in the late 19th century, ADT's messenger business slowly declined in popularity.

[13] In early 1987, the firm Hawley Goodall, owned by Michael Ashcroft, bought the Indianapolis-based Crime Control Inc., the fourth-largest company in the U.S. security market, for $50 million.

This purchase transformed Hawley into the leading security services business in the United States and resulted in the majority of its revenues coming from the North American market.

As a result of the acquisition, Hawley changed its name to ADT Limited and decided to refocus its business around security services.

At the end of 1987, the company sold its North American-based facility services business to Denmark's ISS A/S.

[17][18] In February 2016, Apollo Global Management acquired ADT for nearly $7 billion and merged it with another home security firm, Protection 1.

[28] ADT completed divesting its commercial security, fire and life safety business unit to private equity firm GTCR for $1.6 billion in October 2023.

[29][30] As of March 2019, ADT had nine monitoring centers and a network of more than 17,000 professionals, serving 6 million customers in over 200 locations throughout the United States.

[31][32][non-primary source needed] On July 10, 2023, ADT disclosed that errors were identified in regards to their non-cash goodwill impairment losses associated with their Solar reporting unit and related tax impacts.