Jensen was born in Annan, Scotland to Margaret and Ivar Jensen. She was raised by her mother, a special educational needs teacher. Ashley trained at Queen Margaret University, Edinburgh and was a member of the National Youth Theater.
Jensen's first big television role was as eccentric secretary Rosie McConnichy in the final series of BBC comedy May to December, as a replacement for the character of secretary Hilary. In 1994 she played Heather in Roughnecks, a BBC television series based around the workers on a North Sea Oil platform. In 1996 she played Clare Donnelly, daughter of Glasgow criminal Jo-Jo Donnelly (played by Billy Connolly) in the BBC drama Down Among the Big Boys. She then appeared as Fiona Morris in EastEnders. Jensen had a small role in a Dangerfield episode "Contact" as the mother of a young girl with meningitis. She also appeared inTwo Thousand Acres of Sky on the BBC in 2003 along with Michelle Collins and Paul Kaye.
Jensen starred alongside Ricky Gervais in the BBC Two/HBO television programme Extras as the socially inept Maggie Jacobs. For her work on the first series, she received best television comedy actress and newcomer awards at the 2005 British Comedy Awards. In 2006, Jensen received two British Comedy Awards and a BAFTA nomination for her role in Extras. Her role in the 2007 Christmas Special earned her an Emmy Award nomination. In January 2006 she starred in the short-lived drama series Eleventh Hour on ITV, alongside Patrick Stewart. Later that year she played Steve Coogan's agent in A Cock and Bull Story. In 2007, she provided the narration for the film Taking Liberties, directed by Chris Atkins.
In September 2006, Jensen's first major role on American television came when she took the role of regular character Christina McKinney on the ABC series Ugly Betty. Her character was originally an American, but when she met with the show's producers they liked Jensen's accent and changed the nationality of the character, keeping McKinney British. Production of Ugly Betty moved to New York from Los Angeles in mid-2008, causing a conflict for Jensen, who became a recurring member of the cast.
In January 2009, Jensen starred in No Holds Bard, a BBC Scotland one-off special comedy shown on Burns Night as part of a line-up of special programming to mark the 250th Birthday of Robert Burns. She is the narrator of Channel 4's Embarrassing Illnesses and Embarrassing Bodies. She also narrates Bank of Scotland's current advertisement campaign and Persil detergent adverts for both television and radio. From September 2009 to May 2010, she co-starred with Jenna Elfman in the sitcom Accidentally on Purposeas Olivia. Jensen starred in the BBC comedy Nativity as Jennifer Lore, a Hollywood producer's secretary and the love interest of Paul Maddens played by Martin Freeman.Nativity was released in cinemas on 27 November 2009.
On 21 December 2010, she starred in the one-off/pilot BBC comedy drama Accidental Farmer, playing a London executive who buys a derelict farm with her philandering boyfriend's credit card. In 2011, she voiced Nanette the frog in the animated production Gnomeo and Juliet. She starred in the ITV drama The Reckoning in April of that year. On 6 October 2012, she appeared as a guest on the BBC cookery programme Saturday Kitchen hosted by James Martin.
On 22 August 2014, Sky announced that it had commissioned a television film adaptation of the M. C. Beaton novel Agatha Raisin and the Quiche of Death for Sky1, starring Ashley Jensen as the titular character, Agatha Raisin. She reprises the role 18 months later.
In 2015, Jensen starred opposite Sharon Horgan and American comedian Rob Delaney in Catastrophe, a six-part comedy series for the British Channel 4 that will air in the UK from 19 January 2015.
In 2015, Jensen co-starred with Susan Calman in the BBC Radio 4 comedy series Sisters. Calman also wrote the series.
- Extras (2005-2007) as Maggie Jacobs
- Ugly Betty (2006-2010) as Christina McKinney
- Agatha Raisin (2014-present) as Agatha Raisin
- Catastrophe (2015-2019) as Fran
- After Life (2019-present) as Nurse Emma
- Nativity! (2009) as Jennifer Lore
- How to Train Your Dragon (2010) as Phlegma the Fierce (voice)
- Gnomeo & Juliet (2011) as Nanette (voice)
- Sherlock Gnomes (2018) as Nanette (voice)
- How to Train Your Dragon: The Hidden World (2019) as Phlegma the Fierce (voice)
- Lady and the Tramp (2019) as Jock (voice)
For an up-to-date filmography, see here.