Comcast beat out AOL Time Warner and Cox Communications in the deal valued at $47 billion.
The combined entity, which will be called AT&T Comcast, will have 21 million customers and be based in Philadelphia, Comcast's headquarters.
AT&T Chairman C. Michael Armstrong will serve as chairman, while Comcast President Brian Roberts will be chief executive. Mr. Armstrong will remain chairman and chief executive of AT&T Corp. until the merger closes, sometime in mid-2002.
In winning the contest for AT&T Broadband, Comcast will assume about $25 billion in debt.
T.S. Kelley, director and principal media analyst, Neilsen/NetRatings, said the merger is "not very fruitful in the short-term. It will be very challenging, but the opportunity is great several years from now."