The Federal Communications Commission said the Philadelphia company must clearly ask customers before charging them for new services or equipment and make it easier for customers to fight charges they think are wrong.
"It is basic that a cable bill should include charges only for services and equipment ordered by the customer - nothing more and nothing less," said Travis LeBlanc, head of the FCC's Enforcement Bureau. "We expect all cable and phone companies to take responsibility for the accuracy of their bills and to ensure their customers have authorized any charges."
Comcast said Tuesday that it has been working to improve customer service and that the problems uncovered by the FCC stemmed from "isolated errors or customer confusion" rather than Comcast intentionally mischarging its 22 million cable-TV subscribers.
The FCC's settlement with Comcast, which resolves a two-year investigation, said that some customers complained that they were charged for a service or equipment after they specifically said no to Comcast representatives.
The agency said there were "many" complaints about "hours-long and repeated phone calls" to try to fix the problem and claims of "unhelpful or abusive behavior" from customer-service representatives, including calls getting disconnected.
A Senate investigation this summer criticized Comcast and Time Warner Cable, now owned by Charter Communications, for often failing to refund customers who had been mistakenly charged for cable boxes.
The FCC said Tuesday the settlement represents the largest-ever civil fine for a cable operator. The agency has fined other companies larger amounts, such as a $100 million fine last year against AT&T for slowing cellphone customers' data speeds. The wireless carrier is contesting that fine.
Observers said the fine was unlikely to present much of a financial burden for Comcast Corp., which recorded a profit of more than $8 billion last year.
The Associated Press contributed to this report.