Sieber is more well known for originating the roles of Sir Dennis Gallahad, The Black Knight, and Prince Herbert's Father in the Tony award winning musical Monty Python's Spamalot: The new musical 'lovingly' ripped off of the motion picture "Monty Python and the Holy Grail"', for which he was nominated for a Tony award for Best Performance by a Featured Actor in a Musical. He reprised those roles in the London production. He was seen at the Canadian and North American premieres of Eric Idle and John Du Prez's oratorio "Not the Messiah (He's a Very Naughty Boy)". His other stage credits include Beauty and the Beast, Thoroughly Modern Millie, Into the Woods (2002), Chicago, and Triumph of Love.
Sieber's television credits include regular roles on It's All Relative and Two of a Kind.