The last individual to be granted the award was the conductor Sir John Barbirolli in 1969. Previous recipients include the Queen Mother in 1954.
Paul Richards was given the award by the Borough Council of King's Lynn and West Norfolk because he worked "tirelessly to promote" its history.
The former mayor said he was "pretty humbled and surprised".
The honorary role is the council's highest award but is "purely ceremonial", without legal rights or responsibilities.
Mr Richards is a town guide who has written a number of books on King's Lynn's history and researched the town's role in the Hanseatic League.
He was also a borough councillor and mayor, is involved with a number of trusts and is currently deputy lieutenant of Norfolk.
Sixteen people have been given the honour since it was set up in 1901.
Three organisations have received the freedom of the borough - RAF Marham, the Royal Anglian Regiment and, most recently in 2014, No 42 (King's Lynn) Sqn Air Training Corps.
A town historian has become the first person to receive the freedom of King's Lynn for more than 40 years.