Black is all of 20 years old. And Friday, she made history by becoming the youngest lawmaker in UK history since the 17th century.
Here's what you need to know about the Scot who came from behind and shook the political landscape.
As member of the Scottish Nationalist Party (the SNP), she went up against Douglas Alexander, a long-time Labour Party heavy-hitter. He is a former Cabinet minister and such a shrewd veteran of political campaigns that he was Labour's campaign chief.
He is also a member of the shadow cabinet -- the group of politicians from the opposition party who would head up ministries if in power. If he had been elected, and had Labour come to power, he would have become the new foreign secretary, a position similar to the Secretary of State in the U.S.
But in the end, her 23,548 votes outstripped her opponent -- Alexander ended the night with 17,864 votes -- overturning the veteran's huge majority from the 2010 general election. Alexander has represented the area since 1997, and today marks the first time for 70 years that the constituency has not been a Labour seat.
She's the youngest MP since 1667
In 1667, after being "returned as knight of the shire," Christopher Monck
took his place in Parliament when he was just 13.
Sure, there have been other young parliamentarians, but none as young as Black -- in the last 350 years.
Most recently, former Liberal Democrat leader Charles Kennedy was elected into Westminster at the tender age of 23. But he was three years older than Black is now.
She was into politics even as a kid
Black grew up in Paisley, a working class town in the west of Scotland. From a young age, she would march for causes as diverse as support for a Scottish parliament and opposition to the Iraq war.
She was a vocal supporter of Scottish independence during last year's referendum campaign. She says that she "witnessed first hand the level of poverty and injustice prevalent in our society," touring some of the country's most deprived areas.
The referendum campaign failed, but a passionate, fiery and intelligent campaigner was born.
"Over the two years of that referendum campaign it was a political awakening," she told CNN's Phil Black. "It forced people to engage and think about arguments and try and articulate arguments and look for quality."
She's drawn fire for some of her tweets
A season ticket holder and lifelong fan of her local football club, Partick Thistle, Black has occasionally let her soccer tribalism get the better of her.
During one match against perennial Scottish champions Celtic in October 2013, she tweeted: "I've only just realised -- I really f***** hate Celtic" and "Celtic, yer a joke! #scum."
And like many other Scottish students she's also used social media to boast about her drinking. She tweeted her love of Smirnoff Ice, calling it the "nectar of the gods." She also tweeted about waking up next to a can of beer and a pizza, calling it "a success."
Like a novice, the tweets remained undeleted until after her selection -- at least until the media managed to get hold of screen grabs.
The SNP defended her selection as a candidate, saying, "Mhairi is a dedicated young woman who learned a great deal during the referendum, and wants to do her best for her community as its MP."
She's changed her tune
These slip-ups, along with a speech where she seemed to suggest a desire to physically assault opponents to the referendum, threatened to derail her freshman campaign.
After the referendum result had come in, she told a pro-independence rally that her political opponents' apparently insincere commiserations had made her want to put "the nut on" -- head-butt -- them. She also called unionists "selfish" and "gullible."
Comments like these put pressure on SNP leader Nicola Sturgeon to deselect her as a candidate. Sturgeon stood by her woman, and Black has since adopted a much more humble approach.
She says that she has learned her lesson, telling
Scottish tabloid newspaper the Daily Record, "It's not a figure of speech I will use again. And as far as my remarks about No voters are concerned, I realised myself before the referendum that it was wrong to talk in these terms - I learned and I changed my language.
"If elected, I will be proud to represent all the people of Paisley and Renfrewshire South."
She's got a dissertation to finish
She's a final-year student of Politics and Public Policy at the University of Glasgow. In order to graduate -- and take on her first graduate job in the House of Commons -- she has to finish her dissertation. It's due at the end of the month.
But what'll she do first, a reporter asked her Thursday night? "Sleep," she replied. And then? "Breakfast."