"I knew today was the first day, and I think it's going to be one of the best things for us as a community," said Gloucester Mayor Sefatia Romeo Theken, who worked for 20 years in health care before becoming the city's chief executive in 2015.

Mark Goldstein, the president and CEO of Anna Jaques, said an expansion of the Newburyport hospital will include maintaining current offices in Amesbury and Haverhill while branching out to new markets in Seabrook and Salisbury.

“Each of the organizations joining Beth Israel Lahey Health brings a strong legacy and unique expertise to this new system,” said Ann Ellen Hornidge, chairman of the board of the new merged entity.

But Cormier and other Addison Gilbert officials have said the merger should increase access to care for Cape Ann residents by opening doors to coverage at Beth Israel and other facilities, including Anna Jacques.

The agreement also puts a limit on how much the new system can raise prices and requires the newly merged hospitals to spend $71.6 million over eight years to improve access to health care for low-income and under-served communities.

The merger also comes as Addison Gilbert Hospital has taken additional steps toward expansion, including last year's opening of a new urgent care clinic in the Gloucester Crossing complex, and the boosting its Senior Adult Unit serving patients experiencing sudden changes in mental status and chronic mood disorders from 12 to 20 beds.