John T. Mather Hospital in Port Jefferson, shown in July...

John T. Mather Hospital in Port Jefferson, shown in July 2017, has agreed to join Northwell Health. Credit: Newsday / Thomas A. Ferrara

John T. Mather Memorial Hospital in Port Jefferson has agreed to join the Northwell Health system.

The hospital, which will now be known as Mather Hospital, becomes Northwell’s fifth hospital in Suffolk County.

Mather signed a letter of intent to join Northwell in July. At that time, officials said they would work to develop a final agreement, including gaining the approval of the boards of directors at the hospital and at Northwell. The final agreement will be signed on Thursday and takes effect in January.

Northwell doesn’t expect to make major changes at Mather immediately, said Michael J. Dowling, president and chief executive of the New Hyde Park-based health system.

“We don’t want to disrupt something that’s going well,” Dowling said in an interview earlier this month. “We will spend 12 months identifying what our priorities should be.”

Mather has 12,000 inpatients and 41,000 emergency room cases each year. It has 2,568 employees, according to its website.

Dowling said he didn’t anticipate layoffs.

Mather is the first hospital to join the Northwell system since Peconic Bay Medical Center in Riverhead joined in 2016. At Peconic, Northwell established a cardiac catheterization program in the fall and broke ground on a $60 million critical care tower.

Northwell has committed to making a series of clinical and infrastructure investments at Mather, although it didn’t specify what the investments would be.

Three other hospitals in the county have joined Northwell’s network in the last 23 years: Huntington Hospital in 1994, Southside in Bay Shore in 1996, and South Oaks in Amityville in 2012. Mather is the most recent Long Island hospital to join or affiliate with a larger health system. Southampton Hospital joined the Stony Brook Medicine health system this summer and was renamed Stony Brook Southampton Hospital.

South Nassau Communities Hospital in Oceanside is in talks with the Mount Sinai Health System with the aim of possible affiliation. A South Nassau spokesman said Wednesday those talks are progressing.

With about 63,500 employees, Northwell is the largest private employer in the state. The system operates 23 hospitals, 600 outpatient facilities and has nearly 15,000 affiliated physicians in greater New York. It has about 100 outpatient facilities in Suffolk County.

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