Fielding was born in Poole, England, and grew up in Portsmouth before spending two years boarding at Christ’s Hospital school in Sussex, where she developed an interest in acting.. She also attended Dynamo Youth Theatre, a youth theatre for 11-18 year olds in Havant. She trained at the Guildhall School of Music and Drama; her performances there included Trinculo in The Tempest (directed by Patsy Rodenburg) and Myrrah in Tales from Ovid (directed by Christian Burgess). She graduated from Guildhall in 2006.

In 2008 she appeared in “Firewall”, the second episode of the BBC TV series Wallander. 2009 saw Fielding as a guest on The Bill, playing Rochelle Chapman. In 2010 she appeared in the Doctor Who episode “Victory of the Daleks”, as well as an episode of Midsomer Murders and Comedy Lab. She played Chloe in the Channel 4 sitcom Pete versus Life. In 2016 Susannah played Emma (Murray’s colleague who has a crush on him) in I Want My Wife Back. In 2016, she appeared in “Nosedive”, an episode of the anthology series Black Mirror. Since October 2016, she has had a recurring role in the American sitcom The Great Indoors as Brooke, the ex-girlfriend and current boss of Jack (Joel McHale), who must help him transition from an adventurous reporter to an office worker.

Fielding was previously in a relationship with actor Tom Hiddleston, who appeared with her in an episode of Wallander in 2008. Their relationship ended in November 2011.

Fielding’s first four professional roles in theatre were in four productions at the National Theatre: The Rose Tattoo (2007), Philistines (2007), Much Ado About Nothing (2007) and The Hour We Knew Nothing of Each Other (2008). She received an Ian Charleson Award Commendation for her role as Pietra in Ibsen’s An Enemy of the People at the Sheffield Crucible in 2010 alongside Antony Sher.

Fielding appeared in the 2010 films 4.3.2.1. and 1st Night (originally titled Cosi), for which she learnt to sing opera in Italian. In 2011 she starred in Kill Keith, a comedy horror film featuring Keith Chegwin. She appeared in the 2012 romantic comedy The Knot alongside Mena Suvari and Noel Clarke.

In 2011 she played Portia in Rupert Goold’s production of The Merchant of Venice for the Royal Shakespeare Company alongside Sir Patrick Stewart as Shylock, reprising the role in the production’s transfer to the Almeida Theatre in 2014. In 2012, she was cast as Kim for the UK run of All New People, a black comedy play written by and starring Zach Braff. The play’s UK run included Manchester and Glasgow before culminating in a 10-week run in London’s Duke of York’s Theatre. Fielding played the role of Evelyn Williams in the musical American Psycho at the Almeida Theatre in December 2013.

In 2019, she co-starred with Steve Coogan in This Time with Alan Partridge. A second series of the show was confirmed in 2020, with filming commencing late that year.