First Minister Carwyn Jones marked 18 months of refurbishment of the Grade II-listed Tramshed.
The one-time vehicle repair depot now includes a music venue and restaurants, with a 40-seat cinema to open soon.
Mr Jones said it was "an iconic building, not just for Cardiff but for Wales".
He added: "It's a great example of a building which 20 or 30 years ago would probably have been torn down but has been restored sympathetically as a place to live, to work in, and for entertainment.
"It is the kind of building which will encourage so many businesses, particularly tech and digital."
The business hub - Tramshed Tech - aims to be a workplace for small creative and tech companies and entrepreneurs.
There are also 30 studio apartments, designed as "live-work" spaces.
Part of Tramshed in Grangetown will also include a community room and a dance therapy and yoga studio for disadvantaged and disabled children.
The 1,000-capacity venue opened last October in a first phase and is used for regular concerts and events ranging from darts to food fairs.
Developer Simon Baston, of DS Properties, said it would provide 300 long term job opportunities with the small and medium sized businesses in the "every changing landscape of Cardiff."
The building in Pendyris Street, with its redbrick facade and distinctive windows, was built between 1900 and 1902 and extended in 1923.
Local councillor Ashley Govier said the redevelopment had been a result of hard work and some luck but had not involved any public money.
"It's a fantastic addition to the area and unlike some other parts of the city we've managed to keep an historic building and re-open it to include some community use," he said.
Council leader Phil Bale said in a "very challenging" period for local government, the authority had been able to use one of its land and building assets to create something of "great pride to the local community and to the businesses based here".
A former tram depot near the centre of Cardiff has been officially opened as an entertainment and community venue and digital business hub.