Watson made a commitment in 2010 to register 15,000 new organ donors, just after she underwent her first heart transplant.
She carries a photo of the donor, U.S. Coast Guard fireman Michael Bovill, who died in a traffic crash in July 2010.
Watson received Bovill's heart the day after his death.
She was on her way to hitting her goal until she underwent her second heart transplant last year.
"I was coming up on 14,000, and then I had to stop," she said, "but I did sign up my nurses."
Watson — a national leader in organ donation advocacy — said receiving the Key to the County was special because she is from Rockland and the morale boost was well-timed.
Speaking at the podium Watson told the group of about 40 her recovery from the last transplant has been difficult, but she is not done advocating.
"I'm just a little but older, and physically a little weaker," she said, "but it doesn't stop the mission."
"I don't know of anyone who's done as much for organ donation as you have and will continue to do," Watson's doctor, William Jakobleff said. "From the Montefiore community, we say 'Thank you, Roxanne.'"
At age 71, Watson said she draws motivation from other donors and recipients as well as the national statistics indicate significant shortages of donors.
Data from the Health Resources and Services Administration show the national transplant waiting list is consistently high each year though 13 people on the list die each day.
"Still such a need," Watson said. "You know, over 100,000 people are waiting. We have to eliminate that."