Dunn first appeared on the Channel 4 soap opera Hollyoaks on 12 September 1996, walking into the video shop in the forty-third episode. Mandy came into the village at the age of 14 as a "wannabe supermodel" and quickly formed a friendship with Ollie Benson. In 2003, Dunn played a supporting role in the Brookside DVD spin-off Unfinished Business. In 2004 Dunn appeared in the fitness DVD Hollyoaks Dance workout with Ali Bastian (Becca Dean) and Jodi Albert (Debbie Dean). Dunn left Hollyoaks as a cast regular in 2006.

In September 2007, it was announced that, along with her former Hollyoaks colleague Helen Noble and theatre actor Graham Tudor, she would be teaching students at a new acting school in Liverpool.

Dunn played Gloria in a UK regional tour of the comedy play Boeing, Boeing. The tour started at the Liverpool Playhouse on 18 December 2008 and finished at the Birmingham Rep on 25 April 2009. Dunn appeared in a tour of The Vagina Monologues which began on 7 September 2009 at the Churchill Theatre, Bromley and ended on 8 December 2009 at the Jersey Opera House. In 2010 Dunn played Sally in the UK regional theatre tour of When Harry Met Sally which started on 3 February at the Theatre Royal Wakefield, and ended on 26 June at the Sheffield Lyceum Theatre. When Harry met Sally then played at the Grand Canal Theatre in Dublin, Ireland for one week ending on 3 July 2010.

Dunn has since returned to Hollyoaks five times. In February 2007 for a two episode stint for the funeral of Mandy's best friend, Becca Dean. She returned in 2008 for six months, leaving in December after the disastrous wedding of Warren Fox (Jamie Lomas) and Louise Summers (Roxanne McKee). In November 2010 Dunn returned to Hollyoaks with a one-year contract alongside Jamie Lomas, leaving in September 2011. Dunn reprised her role as Mandy for a guest stint in the spin-off Hollyoaks Later in October 2013. On 2 June 2017, it was announced that Dunn would reprise her role as Mandy in the main show, with her return scenes due to air in July.