Seagrove was born Jennifer Ann Seagrove in Kuala Lumpur, Malaya (now Malaysia) in 1957, to British parents, Pauline and Derek Seagrove. Her father ran an import-export firm, which afforded the family a privileged lifestyle. When Seagrove was less than a year old, her mother suffered a stroke, and was unable to care for her. Seagrove attended St Hilary's School in Godalming, Surrey, England from the age of nine.

Seagrove starred alongside Rupert Everett in the Academy Award-winning short film A Shocking Accident (1982), directed by James Scott. Her first major film appearance was in Local Hero (1983) in which she played a mysterious Environmentalist with webbed feet. Roles in a number of films including Nate and Hayes (aka, Savage Islands, 1983) opposite Tommy Lee Jones and Appointment with Death (1988) followed. One of her lead starring roles was in The Guardian (1990), directed by william Friedkin, in which she played an evil babysitter. In 2017, she played the lead role in Another Mother's Son, starring as Louisa Gould, a member of the Channel Islands resistance movement during World War II, who famously sheltered an escaped Russian slave worker in Jersey. She was later gassed to death in 1945 at Ravensbrück concentration camp.

Seagrove was previously married to British and Indian actor Madhav Sharma from 1984 to 1988 and then dated film Director Michael Winner until 1993.

Jenny Seagrove's theatre work includes the title role in Jane Eyre at Chichester Festival Theatre (1986); Ilona in The Guardsman at Theatr Clwyd (1992); and Bett in King Lear in New York, again at Chichester (1992).

With John Thaw she guest starred in the episode "The Sign of Four" (1987) of the series Sherlock Holmes. She guest starred in episodes of Lewis ("The Point of Vanishing", 2009) and Identity ("Somewhere They Can't Find Me", 2010). A few years later, she appeared in the series Endeavour (the prequel to the Inspector Morse series), in the episode "Rocket" (2013).

Most of Seagrove's filmed work since 1990 has been for television. Between 2001 and 2007, she appeared as QC Jo Mills in the series Judge John Deed. She was the subject of This Is Your Life in 2003 when she was surprised by Michael Aspel.

She played opposite Tom Conti in Present Laughter at the Globe Theatre (1993); Annie Sullivan in The Miracle Worker at the Comedy Theatre (1994); Dead Guilty with Hayley Mills at the Apollo Theatre (1995); Hurlyburly for the Peter Hall Company when the production transferred from the London Old Vic to the Queen's Theatre (1997); co-starred with Martin Shaw in the Parisian thriller Vertigo (Theatre Royal Windsor October 1998) and then with Anthony Andrews (also Windsor,1998).

Seagrove, along with Simon Cowell, presented Wildlife SOS (1997), a documentary series about the work of dedicated animal lovers who save injured and orphaned wild animals brought into their sanctuary.

In 2000 she appeared in Brief Encounter at the Lyric Theatre; followed by Neil Simon's The Female Odd Couple at the Apollo (2001). Again at the Lyric Theatre in 2002 she played the title role in Somerset Maugham's The Constant Wife, followed by a revival of David Hare's The Secret Rapture in 2003, and The Night of the Iguana two years later in 2005.

She is known for her role as the character of Jo Mills in the long-running BBC drama series Judge John Deed (2001–07). Her credits as a voiceover Artist include a series of Waitrose television advertisements.

In December 2007, she played Marion Brewster-Wright in the Garrick Theatre revival of Alan Ayckbourn's dark, three-act comedy Absurd Person Singular.

In 2008, she and Martin Shaw starred in Murder on Air, at the Theatre Royal Windsor.

In 2011, she once again starred alongside Martin Shaw in The Country Girl at the Apollo Theatre, playing the part of Georgie Elgin.

In 2014, she performed a duet alongside singer Peter Howarth called The Main Chance, as part of a cause for the Mane Chance Sanctuary which Seagrove founded.

In 2015, she and Martin Shaw starred in a radio adaptation of Brief Encounter at the Theatre Royal Windsor, playing the parts of Laura Jesson and Alec Harvey.

As of 2017, Seagrove is currently playing Chris MacNeil in The Exorcist until March 2018, at the Phoenix Theatre.