The daughter of the former Ryder Cup captain Bernard Gallacher and Lesley Gallacher, Kirsty Gallacher was born in Edinburgh, Scotland. When her father got the job of professional at Wentworth Golf Club, they moved south to Virginia Water, England, when she was 18 months old. Gallacher and her younger brother and sister were raised on the Wentworth Estate, where their close neighbours included Bruce Forsyth. She was educated at Coworth Flexlands and St George's School in Ascot, Berkshire.
Wanting to be a fashion journalist, Gallacher studied at the London College of Fashion in journalism and promotion. Approached by Mark Sharman, then deputy head of sport at Sky, at a Ryder Cup dinner, she began working as a production assistant in 1996, before moving on to the position of editorial assistant on Sky Sports.
She got her presenting break on Sky Sports News in 1998, and since then has been associated with many other sports shows such as 90 Minutes, Soccer Extra, Kirsty and Phil (on BBC Radio 5 Live) and Soccer AM.
From 2000 until 2004, Gallacher hosted the Sky1 show Kirsty's Home Videos, a satellite equivalent of ITV's You've Been Framed! in which viewers send in home videos to be screened. In 2002, she presented the morning show RI:SE, and in 2004 began the show Simply the Best. Gallacher presented the All Star Cup on Sky1 in 2005 and on ITV in 2006. She has made guest appearances on They Think It's All Over and A Question of Sport. Gallacher presented Ant vs Dec – the popular strand on Ant & Dec's Saturday Night Takeaway.
In 2005, Gallacher won the third series of the popular Channel 4 reality television series The Games. She then presented the following years alongside Jamie Theakston. In December 2005, she released a DVD entitled Body Sculpt with Kirsty Gallacher in which she shows a variety of workouts. In July 2006, she co-presented the BBC reality TV show Only Fools on Horses.
In 2008, Gallacher presented the new version of Gladiators on Sky1 alongside Ian Wright. She did not return for the second series.
On 30 August 2008, Gallacher appeared on Who Wants to Be a Millionaire? with Duncan Bannatyne.
She has also guest presented GMTV with Lorraine in August 2010 for two weeks and Lorraine on ITV Breakfast.
In May 2011, Gallacher returned to Sky Sports to present for Sky Sports News.
On 27 August 2015, it was confirmed that Gallacher would be taking part in the 2015 series of Strictly Come Dancing. On 5 September 2015, it was announced Gallacher would be paired with professional dancer Brendan Cole for the series. The couple were eliminated on week 6; finishing eleventh.
In December 2016, Kirsty Gallacher suffered an attack live on air which included the slurring of speech and her collapse. She was rushed to hospital for tests and found to have been suffering from "extreme exhaustion and a viral infection".
Gallacher was nominated for Best Newcomer at the Royal Television Society Television Sports Awards in 1998. She won Satellite/Digital TV Personality at the 2002 Television and Radio Industries Club Awards, and Kirsty's Home Videos won her the following year's Satellite/Digital Programme award in 2003. She also won a "Platinum Award" for Best Entertainment Programme at the 2002 Houston International Film Festival.
Gallacher is the cousin of golfer Stephen Gallacher. At the end of February 2013 she was named patron of Hibernian ladies.
In August 2014, Gallacher was one of 200 public figures who were signatories to a letter to The Guardian opposing Scottish independence in the run-up to September's referendum on that issue.
On 12 August 2017, Gallacher was stopped by police in Eton near Windsor Castle, breathalysed at the roadside and arrested. She pleaded guilty at Slough Magistrates' Court for being over three times the legal drink drive limit the morning after a night out. On 4 September 2017, Gallacher was banned from driving for two years, ordered to serve 100 hours of community service, pay £85 prosecution costs and an £85 victim surcharge.