Ritchie was cast as Molly, the child lead in the 1986 Nine Network–PBL mini-series Cyclone Tracy, starring Chris Haywood and Tracy Mann. She started working with Home and Away in 1987, at age 8. Production began in July 1987 (shortly before her ninth birthday) and it premiered on the Seven Network on 17 January 1988. In 2006, Ritchie played “Nicole” in the ABC film Stepfather of the Bride. Whilst promoting Home and Away in the United Kingdom in 2006 with Mark Furze and Jodi Gordon, Ritchie appeared on The Friday Night Project and Loose Women, and appeared on interviews for Five and Five Life.

In 2006, Ritchie was a contestant on the celebrity singing competition It Takes Two, partnered with Troy Cassar-Daley. The pair placed fourth in the competition. She co-hosted the second series of It Takes Two with Grant Denyer in 2007. In 2007 Ritchie joined Nova radio to co-host the drive show with comedian Akmal Saleh, replacing Matthew Newton. She was replaced in May by New Zealand comedian Cal Wilson.

In September 2007, Ritchie announced that she would be leaving Home and Away in mid-December. Her final scenes were filmed on 13 December 2007, and she last appeared on Australian screens as Sally Fletcher on 3 April 2008. Her UK demise with the same episode occurred not long after the Australian, with her departure being screened on UK terrestrial television on 12 May 2008.

On 14 January 2008, Ritchie began work on the Nova 96.9 breakfast show with Merrick & Rosso. Shortly after the 2008 Logie Awards Ritchie did an interview with Rove McManus on Network Ten, her first interview with another network since leaving Home and Away, and her first television appearance since leaving the show. On 16 October Ritchie appeared on pay-TV, on The Merrick & Rosso Show.

Ritchie started dating St George Illawara Dragons player Stuart Webb in 2008. On 9 September 2009, it was announced that they were engaged. On 25 September 2010 Ritchie married Webb in a countryside outdoor wedding at Quamby Estate in Tasmania.

In November, it was revealed that Ritchie would appear in the second series of Underbelly, as Judi Kane, the wife of slain 1970s standover man Les Kane. The series screened in 2009. This was her first TV drama role since leaving Home and Away. Throughout 2009, Ritchie continued her radio role on the Merrick & Rosso and Kate Ritchie Show.

In 2009 Ritchie became the face of Vaseline. It was announced on 7 November 2009 that Ritchie would host the 2009 ARIA Awards on 26 November, alongside actor and fellow Underbelly cast member Gyton Grantley.

On 10 November 2009, Ritchie announced she would be leaving Nova 96.9’s breakfast team and returning to acting. In January 2010 it was announced that Ritchie had been cast with a lead role in Nine’s new police drama Cops L.A.C. The series did not rate well and was cancelled after the season finale.

In 2012, Ritchie narrated the Australian version of the reality series Don’t Tell the Bride, which aired on Ch-10. In early 2013, it was announced that Ritchie was returning to Home & Away, reprising her role as Sally Fletcher.

In 2013, Ritchie announced via Twitter that she would be hosting the Nova FM drive show with Tim Blackwell and Marty Sheargold in 2014, following the departure of Meshel Laurie, who went on to host Breakfast on Nova 100 in Melbourne.The show won three major ACRA awards. In 2015 the team won Best Networked Program at the Australian Commercial Radio Awards, and the Best On Air team in both 2016 and 2017.

On 17 August 2014, Kate gave birth to daughter, Mae Webb.

On 25 November 2015, Ritchie became the Godmother of the Cruise liner Pacific Eden, which is from the P&O Fleet.

In 2016 Ritchie released a children’s book illustrated by Hannah Sommerville called I Just Couldn’t Wait To Meet You. Her second book, It’s Not Scribble to Me, was illustrated by Jedda Robaard and published by Random House Australia in 2018.

In June 2016 Ritchie was announced as the brand ambassador for QV Skincare.

Ritchie was to host the retrospective 50th anniversary of Play School in July 2016.

On 7 November 2019, Stuart Webb agreed to an AVO which was applied for by a police constable on behalf of Ritchie following a domestic incident on 17 October. The AVO prevents Webb from stalking, intimidating, assaulting or threatening, as well as approaching or being in the company of Ritchie “for at least 12 hours after drinking alcohol or taking illicit drugs”.