The idea was almost as old as the colony. As early as 1814, the convict architect Francis Greenway is said to have proposed a harbour crossing to Governor Macquarie, later writing that such a bridge would "reflect credit and glory on the colony and the Mother Country." Designs came and went for a hundred years, including a graceful steel cantilever that won a 1902 competition before an economic slump killed it. The man who finally willed the bridge into being was John Bradfield, appointed chief engineer in 1914. He championed the project for decades and lent his name to the Bradfield Highway that runs across the deck. The detailed design and construction fell to the British firm Dorman Long of Middlesbrough, whose engineers drew on their own recently built Tyne Bridge in north-east England.

The arch is a single steel span of 504 metres, its summit rising 134 metres above the water, making it the tallest steel arch bridge in the world. On a hot Sydney day the steel expands enough to lift the crown of the arch by 18 centimetres. Holding it all together are roughly six million hand-driven rivets, the last hammered home on 21 January 1932. Riveting was the proven technique of the era; structural welding was not yet trusted for a job this size. The rivets were heated red-hot, dropped into place, and flattened with pneumatic guns, and the deafness many workers suffered in later life was blamed on that relentless noise. The four granite pylons are pure theatre, with no structural purpose at all. They were added only to reassure a nervous public that the great arch would not fall down.

Building the bridge through the depths of the Great Depression was brutally dangerous work, and it came at a human cost. Sixteen men died during construction, though only two of them from falling off the bridge itself. Many more were injured handling the scorching rivets and working at heights without the safety equipment a modern site would demand. Some 800 families were displaced when an estimated 469 buildings on the north shore were demolished to clear the approaches, with little or no compensation paid. Far to the south, around 250 stonemasons and their families relocated to Moruya, where they quarried the granite for the pylons. Photographer Henri Mallard spent 1930 to 1932 documenting the riggers and ironworkers at close quarters, leaving a haunting record of ordinary men doing extraordinary, perilous work in desperate times.

The bridge was nicknamed the Iron Lung, because the work kept thousands of Depression-era labourers in jobs when little other work existed. Its full cost of 6.25 million Australian pounds was not paid off until 1988. Today it carries trains, cars, cyclists and walkers between Dawes Point in The Rocks and Milsons Point on the north shore, and since 1998 the company BridgeClimb has guided visitors up and over the eastern arch on three-and-a-half-hour climbs. The structure has become the canvas for the nation's grandest moments: the dazzling New Year's Eve fireworks that stream from its arch, the word "Eternity" spelled out across it at the millennium in tribute to the illiterate chalk-writer Arthur Stace, and the Walk for Reconciliation in May 2000, when somewhere between 200,000 and 300,000 people crossed it on foot.