Armstrong was born and raised in Galashiels. She started her career in local radio, before joining the BBC news team in Manchester in 1983. She then moved to Border Television as a news reporter, before becoming a regular newsreader and presenter for the nightly regional news programme Lookaround. In March 1987, she switched to ITN as a reporter/newscaster on News at Ten, News at 5:45 (later News at 5:40) and other bulletins. On the reporting side, she covered the Lockerbie air disaster and produced a series on aids orphans in Africa. In 1993, she was one of the launch team on the ITV breakfast station GMTV, but left after only a few months.

She presented the breakfast news programme on BBC World before rejoining ITV Border as a main presenter of Lookaround, as well as a presenter and producer on a number of regional programmes for the station, including River Journeys and Fiona on Fishing. She has fronted antiques, political, cooking and lifestyle programmes for the ITV regions and has made fishing programmes for Sky Television.

In February 2009, ITV Tyne Tees & Border launched, replacing ITV Tyne Tees and ITV Border. Armstrong was announced to be a Features Correspondent for the service on a freelance basis. Her work took her to Cambodia to report on the work of landmine charities.

Armstrong returned to network television news on 15 February 2010 as a cover presenter for the BBC News Channel.

Armstrong specialises in Scottish clan history. She has made more than 20 films on Scottish families, and in 2006, set up the first Border Reiver Trail in the south of Scotland.

She writes for magazines and newspapers, including columns for Scottish Field magazine and the American-based Scotbanner newspaper. She has written two fishing books and two cook books.

Her hobbies are fishing, cooking and clan history. She is married to a Scottish clan chief Sir Malcolm MacGregor of MacGregor.