Flames threatened the SS South Steyne after fire broke out on a barge in Berrys Bay about 7.15 pm last night.
Port Authority of NSW and Fire and Rescue NSW crews worked together over several hours to put the fire out.
An FRNSW said they were in the final phase of mopping up before about 11 pm.
He said there was "minimal impact" from the fire to the SS South Steyne and the old wharf it was moored to.
The grand old boat arrived in Sydney in 1938 and carried out the Manly ferry run until the 1970s, as well as 20 years running Sunday ocean cruises out to Broken Bay.
It was the last steam ferry operating in Australia when it was pulled from service and spent some years in Melbourne before returning to Sydney in the '90s, according to the NSW Office of Environment and Heritage.
Its heritage listing calls the boat the best known of the Manly ferry line, highlighting the role the vessels played in Sydney's suburbanisation.
"It epitomised the Manly ferry as part of Sydney's image and its popular urban culture; and remains, like the Harbour Bridge, a powerful piece of Sydney imagery," the listing notes.
"It is held in high esteem by the local community and remains in the collective memory of the nation."
Police established a crime scene and were sending specialists to investigate this morning.