[3][4] Mongoose, his first original stage play, was performed at the Southwark Playhouse in 2003 (directed by Thea Sharrock) and later at the Assembly Rooms, Edinburgh and the Trafalgar Studios, London.
[5] Harness went on to write several other single films for BBC4, including a biopic of Frankie Howerd, Rather You Than Me, starring David Walliams and Rafe Spall, which was part of the Curse of Comedy season, as well as the Spanish flu drama, The Forgotten Fallen, starring Bill Paterson, Mark Gatiss and Charlotte Riley, which dealt with the efforts of a medical team in Manchester to combat the disease in 1918.
[6] City of Vice, a drama series about the Bow Street Runners and the birth of the English police, starring Ian McDiarmid and Iain Glen as Henry Fielding and his brother John, was shown on Channel Four in 2008.
[4] He wrote five feature length films for the series: The Dogs of Riga, Before the Frost, a new story developed by Henning Mankell and Harness called An Event in Autumn, and the two-part finale, A Lesson in Love and The Troubled Man.
[13] Harness also adapted the fourth Jackson Brodie book, Started Early, Took My Dog for the second series of Case Histories, starring Victoria Wood, which filmed in Summer 2012.
[14] Harness also wrote and executive produced on the BBC crime drama series McMafia, based on Mischa Glenny's book about the Russian Mafia, which starred James Norton.
[20] In 2019, he wrote a new miniseries adaptation of The War of the Worlds for BBC One, produced by Mammoth Screen and starring Rafe Spall, Eleanor Tomlinson and Robert Carlyle.