Marsh has appeared several times in the BBC series "Doctor Who". She first appeared alongside William Hartnell in the 1965 serial "The Crusade" as Lady Joanna, the sister of Richard I (The Lionheart). She returned later that year as companion Sara Kingdom in the 12-part serial "The Daleks' Master Plan". Marsh reprised the role in the audio plays "Home Truths" (2008), "The Drowned World" (2009), "The Guardian of the Solar System" (2010), "The Five Companions" (2011), "The Anachronauts" (2012), "An Ordinary Life" (2014) and "The Sontarans" (2016). She also appeared in the 1989 television serial "Battlefield" as Morgana Le Fay, as well as the 2007 audio play "The Wishing Beast". She made an un-billed cameo appearance in the 2013 docudrama about "Doctor Who", "An Adventure in Space and Time".