Scores of visitors gathered at the Grade-I listed town centre building on Saturday, July 8, to enjoy the Steampunk Decadence event.
The festival showcased an unusual mix of colourful Victorian costumers with steam powered gadgetry.
Visitors were invited to take a look around an array of stalls selling goods from clothing to gifts to trinkets and memorabilia.
There was also live music and an exhibition giving members of the public chance to immerse themselves in Doncaster's famous steam and rail heritage.
Steampunks also enjoyed tours around the Mansion House, one of only three in the country which dates back to the Georgian era.
Hilary Sedgewick, of the White Rose Yorkshire Steampunks, who co-organised the event with Visit Doncaster, said: "Steampunk is a neo-Victorian science fiction fantasy contrivance and the phrase ‘Steampunk’ was first coined in the 1980’s.
"Since then a whole culture has grown consisting of men and women who enjoy the formalities and dress of the Victorian era as well as the imagination and gadgetry of steam."
The fun also spilled out onto the streets as additional stalls were sited in the outdoor market along with additional displays at The Masons Arms, the Marketplace Alehouse and Deli and The Queen pubs.
While several steampunk events have been held in Doncaster in previous years, this was the first one hosted at the Mansion House.