The news was revealed via this two-minute scene which sees Mackie verbally-sparring with Peter Capaldi and facing off against the Daleks! It also gives us her name - Bill!
The 28 year-old actress has previously been a rising star of the theatre, with only one film and TV credit apiece to her name in terms of screen acting. Landing a role as one of the leads in a globally-popular brand is quite the baptism of fire, then!
Mackie has described Bill as "cool, strong, sharp, a little bit vulnerable with a bit of geekiness thrown in." Sounds like a promising description for a character to me...
If I might put my fan hat on for the moment (it's TARDIS blue with the Doctor Who logo on the front, if you must know), from what we can gather from this clip it seems that Bill will err on the Clara side of companions - those that challenge the Doctor a little more than the likes of Rose or Amy. This might seem an odd move, as you would think a completely different type of character would be appropriate.
However, a companion who knows nothing about aliens and time travel and whatnot again will certainly be refreshing after three series with Jenna Coleman in the role. Change is a key part of the show, and I always look forward to those first few episodes of a series where the companion gets to grips with travelling with the Doctor.
Here are two promo images that were also released. Do we think they look like a good pair?
While this announcement is hugely exciting, it is sad to think that this scene is the the only Who we will get till Christmas. It looks like we will be watching this snippet a fair few times over the coming months...