He has appeared in numerous productions including Fat Christ at the King's Head Theatre, Charlie and Henry at the New End Theatre, 1966 World Cup Final on tour around the UK, and Pawnography and Le Jet De Sang at The Rose (theatre) in London 2007, part of their first residency in 500 years.

Downie's first television appearance was when he starred as Street in Out of Tune, a British children's TV sitcom which was shown on CBBC from 1996 to 1998, alongside James Corden and Jane Danson. Downie's work in television includes Hollyoaks 1998 - 2001,BBC Four's Micro Men 2009, The Legend of Dick and Dom 2009 on CBBC, M.I. High 2008 for the BBC, New Tricks 2004 for the BBC, Keen Eddie 2004 which aired on the Fox Network, Judge John Deed 2001 for the BBC, Doctors in 2002.

He appeared as himself in To Me... To You... in 1998. In 2010 and 2011 he starred with Sam Heughan in an award-winning series of commercials for Tennents Lager in the UK.

Other films include Homicide: Division B written and directed by Abner Pastoll and starring Edward Peel. It premiered at the Cannes Film Festival and has been in competition at several high profile international film festivals including Newport Beach Film Festival, Tiburon Film Festival and the final Siena Short Film Festival in Italy. It was selected by Jason Reitman as a finalist in the Project Direct! film competition. He also starred in the 2004 film Shooting Shona, and as H.G. Wells in the 2010 short film A Great Mistake, produced by February Films, with Abner Pastoll directing both films.

As a Writer Downie had his first theatre piece commissioned in 2008, The Dead Moon which toured around the UK and played at the Aldeburgh Festival, the first non-operatic piece to ever be performed there. He started writing comedy later that year with the web-series The Real Brian Blessed for ComedyBox.tv. In 2009 he was a finalist in the New York Screenwriters' Challenge with 'The Robin Wins The Spring' and at the Atlanta Skyfest II Festival for his script The Automata Circus. Since then his theatre work has included The Curse of Elizabeth Faulkner for the Off Cut Festival 2010 in London, which was also performed at the King's Head Theatre, The Revenge of Anubis for BBC Radio Devon recorded at Ealing Studios, 'The Story Project 2 – Love, Lies and London' for The Southwark Playhouse, A Portrait of Maureen Flange for the Etcetera Theatre, and Theatre in a Car for the opening of the London Bridge Festival in 2010.

He appeared as the Duke of Gloucester in the 2010 83rd Academy Awards-winning film The King's Speech.

His TV work includes starring as Yates opposite David Jason in the BBC series The Royal Bodyguard, both series for Sky Atlantic of This is Jinsy. The first series was nominated in 2011 for the British Comedy Awards for Best Sketch Show In 2013, he played the lead regular of Tim Curtain in Father Figure.

Since 2012, Tim has played the regular character of Danny Bear on the award-winning cult series Toast of London.

In 2014, he appeared in the award-winning Paddington, as Explorer Montgomery Clyde.

In 2015 he appeared in Hampstead Theatre’s acclaimed Matchbox Theatre written by Michael Frayn

Downie portrays Kit Marlowe in Ben Elton's ongoing sitcom Upstart Crow, which started in 2016.

Recent film credits include War Machine (film) with Brad Pit, 6 Days (2017 film) and The Mercy