Minister of Regional Development Kiri Allan and Gore Mayor Ben Bell unveiled the plaque marking the opening.
In 2020, the Government announced it would give $3 million from its "shovel ready" fund towards the redevelopment of the James Cumming Wing, including a new library.
Mataura Valley Milk (MVM) also gave $958,000.
The original building was built by Gore firm Jones and Cooper and one of the original owners of that company, Fred Cooper , now aged 101, was at the opening.
Jones and Cooper, now run by Mr Cooper’s son, Peter, completed the redevelopment.
Gore District Council arts and heritage curator Jim Geddes said the forebears of the town would be amazed by the facility but not surprised.
"Our district’s history is redolent with valiant crusades focused on endowing our town with first rate educational and cultural facilities."
Ms Allan said the Government was "privileged" to be able to contribute to the cost of the redevelopment.
"It’s about seeing places like Gore as being incredibly vital, the backbone of New Zealand’s economy and culture."
She was glad to visit Gore, as her grandparents, Margaret and David Allan, had lived in the town and her father, David, had spent many hours in the library as a child.
Mr Bell said when James Cumming left money in his will for the building of the original facility he "set in place a legacy of togetherness".
The library had been a controversial issue in the past few years but it was an accomplishment and credit to those involved, he said.
MVM chief executive Bernard May said while his family did not have much money when he was a child they were rich in other ways because every week his mother took him and his brother to the library.
"It absolutely blew my mind when I was a young kid that I had access to all this knowledge and all of this history.
Hokonui runanga kaitoko mātauranga Jo Brand said the community was "truly spoilt" to have such a facility.
It was a space which different generations could use which in these times was not common, Ms Brand said.
"A true taonga for all people in Maruawai that will serve us well into the future."
Community members Valmai Redhead and Karen Bellew were impressed with the makeover.
"The library is like the living room of the city and this is a living space that will take this community into the future for many years," Mrs Redhead said.
"I can see children loving that space — it’s beautiful," Ms Bellew said.
