The SNP win - in a hitherto staunchly Labour seat - was a massive boost to the Nationalist cause.
Margo, then a 30-year old trainee physical education teacher from Lanarkshire, was dubbed the Blonde Bombshell of Scottish politics.
Her stint in Westminster was short-lived. She lost the seat at the General Election four months later but her impact was immense.
The 1974 Election saw 11 SNP MPs returned to Westminster. Although she was not one of them she was voted in as Deputy Leader of the Party, a post she held for five years.
Her politics were always left of centre and she was a forceful campaigner for Scottish independence.
She married former Labour MP JIm Sillars who switched to the SNP and, by coincidence, won the Govan seat for the Nationalist in 1988 - 15 years after his wife's victory.
Margo's exile from Westminster saw her become a regular newspaper columnist and television and radio presenter,
In 1982 she resigned from the SNP in protest at the proscription of the Socialist 79 Group, of which she had been a member.
But she returned to the fold in the 1990s and was elected as an SNP member to the Scottish Parliament in 1999, representing the Lothians.
Her support for Alex Neil in the 2000 leadership contest saw her marginalised and her decision to stand as an Independent saw her expelled from the Party in 2003.
Since then she has stood successfully under the Independent banner at three elections and in 2007 lost out to Alex Fergusson in a bid to become Parliamentary Presiding Officer.
But the last 18 years of her life have been dominated by her battle against Parkinson's Disease and her bid to introduce a right-to-die Bill at Holyrood.
She said of her campaign: "The possibility of having the worst form of the disease at the end of life has made me think about unpleasant things.
"I feel strongly that, in the event of losing my dignity or being faced with the prospect of a painful or protracted death, I should have the right to choose to curtail my own, and my family's, suffering."
Her bill has twice been defeated at the Parliament.
Margo was born in 1943 and lived in the East Kilbride area during her childhood. When she was 22 she married her first husband Peter MacDonald and the two ran a pub in Blantyre for a few years.
She has four children, one of whom Petra is married to Craig Reid of the Proclaimers.