At age 15, she joined her older brothers, Seth and Mojo, on a tour of Europe with the Turbozone International Circus Company as an acrobat.

Some of Fellner's multiple roles on television include her appearance on the cult comedy TV show, The Fast Show (2000), which gave her the opportunity to play various roles with Paul Whitehouse, Charlie Higson, Simon Day, and the rest of the cast, My Hero (2002), The Line of Beauty (2006), The Eustace Bros (2003), Hollyoaks (2006), where she was directed by her now-husband Adrian Vitoria, Casualty (2007–2009), and The Trip To Italy (2014) as Lucy, among others.

After the success of The Trip To Italy, which premiered at the Sundance Film Festival, Fellner then made a second movie with Michael Winterbottom called The Face of An Angel (2014), about the Amanda Knox trial, which should be released in the US in 2015.

Coming up, Fellner will appear in the movie 'Bus 657' (2015 – currently in post production) with Jeffrey Dean Morgan, Robert De Niro, and Kate Bosworth.

Fellner has also appeared in multiple short films, including a modern-day adaptation of Hamlet, All Our Sins Remembered (1998) as Emily Brahinski, Incorporated (2010) as Clarice, and Hit Girls (2011) as Emma.