The rapper revealed at the Elle Style Awards in London on Monday (February 13) that the pair are keeping their romance casual for now.
"I don't want to be too tied down or get into anything serious too fast," The Sun quotes him as saying. "But Millie is really fit."
The At Your Inconvenience performer split from girlfriend Candy McCulloch in November.
Meanwhile, Mackintosh separated from her Made In Chelsea co-star Hugo Taylor in 2011.
She famously exposed his affair with her best friend Rosie Fortescue during the E4 show's second series last year.
> Professor Green: 'I'm no longer scared of being happy'
Watch Professor Green in the video for 'Never Be A Right Time' below: