The idea began with an act of imperial nostalgia. In 1897, Borough Councillor F.M. Spurdle proposed that Queen Victoria's record-breaking reign deserved a permanent monument, and Mayor Alexander Hatrick agreed. The Wanganui Opera House Company offered 4,000 of its 5,000 one-pound shares to the public, and the money flowed in. A design competition followed, with the winning architect promised five percent of the total construction cost. Stevenson, working from Wellington, beat out the field. His plans, six meticulous sheets showing basement, ground floor, and dress circle levels alongside cross-sections and elevations, survive today in the Whanganui Regional Museum. Copies hang in the opera house's bar lounge, where patrons can study the building's bones between acts.

The Wanganui Herald praised the new building on its completion in 1900: an auditorium measuring 60 feet square, seating over a thousand, with cast-iron-framed seats whose upholstered backs tipped up to let audiences shuffle past. Premier Richard Seddon himself performed the official opening on 9 February 1900, though the first actual performances, A Sporting Life and The White Heather by the Bland Holt company, had already taken the stage a month earlier. Eighteen cast-iron pillars, painted to imitate marble, held up the dress circle from below. They looked magnificent but blocked the view, and audiences learned quickly which seats to avoid. The circle wrapped around to the proscenium on each side, creating what one observer described as an unusual intimacy between actors and audience. Ten large windows lit the space by day; at night, a 16-horsepower Crossley Brothers engine powered the electric lights.

The opera house has survived more threats than most buildings its age. A sweet stall fire closed the theatre for months in 1937. Another fire struck the annexe in 1978. In 1995, an arson attempt would have succeeded had it not been for a sprinkler system installed just four years earlier with a 51,000-dollar Lotteries Commission grant. When the stage needed replacing in 1993, seventeen volunteers showed up to lay 3,500 lineal metres of matai timber and hammer in 15,500 nails. The old flooring did not go to waste; it was reused in the foyer of the Whanganui Riverboat Restoration and Navigation Trust's centre on Taupo Quay.

Heritage New Zealand awarded the opera house Category One status in 1996, recognizing it as the country's last Victorian theatre. Three years later, centennial celebrations brought a royal charter and a new name: the Royal Wanganui Opera House, later updated to reflect the Whanganui spelling. Today the 830-seat theatre hosts everything from grand opera and orchestral concerts to school events and fashion shows. Wedding ceremonies take place on the same stage where Bland Holt once performed. But the building's age brings challenges too. In 2015, the Whanganui District Council determined the structure is earthquake-prone under the Building Act 2004, a designation that puts the theatre's future in the hands of engineers and preservation advocates alike.