Sydney's skyline has grown taller and wider over the past 120 years, as the old gets replaced with the new.

But one grand old lady has stood the test of time.

The Queen Victoria Building, nestled in the heart of the CBD, was nearly demolished but on Tuesday celebrates 120 years since it first opened its doors.

The now-treasured shopping destination was built as monument to the long-reigning monarch in 1898 and is now an icon of Sydney.

More than 33 million people walk through the doors of the Romanesque-style building each year - many of them tourists - stunned by its architecture.

"The QVB is a quintessentially Sydney shopping destination which has for generations been part of the fabric of the CBD," Daniel Sutton from retail property group Vicinity Centres told AAP.

Mondial Pink Diamond Atelier has been a retailer in the QVB for 25 years with director Michael Neuman describing the building as "one of the world's grandest" shopping precincts.

"As the city around it changes, the QVB stands gleaming and beautiful in the middle of the CBD," he told AAP.

The doors to the QVB were opened in 1898 by the Mayoress of Sydney, Francis Harris, using an intricately inscribed golden key.

As part of the building's 120th birthday celebrations, the QVB has begun to search for the missing heirloom.

Sydney Lord Mayor Clover Moore, the QVB original town crier Bill Wallace and QVB architect Ross Gardener will join the celebrations and the launch of a new art installation in the building on Tuesday.