On June 1, 2007, Sieber sang tenor in the world premiere of Eric Idle's Not the Messiah (He's a Very Naughty Boy) in Toronto.

On January 31, 2014, it was announced that Sieber would be joining the company of Matilda the Musical on Broadway as Miss Agatha Trunchbull starting performances on March 18.

[5][6] On March 4, 2024, it was announced that Sieber would join the cast of Death Becomes Her as Ernest Menville in its preview run at Chicago's Cadillac Palace Theatre.

From 2003 to 2004, Sieber returned to ABC for It's All Relative, in which he and John Benjamin Hickey played Simon Banks and Philip Stoddard, same-sex parents to Liz Stoddard-Banks (Maggie Lawson); other co-stars in the series included Reid Scott, Harriet Sansom Harris, and Lenny Clarke.

In 2007, Sieber portrayed an eccentric zoologist for NBC comedy pilot Wildlife, which was not ordered by the network.

[8] In 2010, Sieber led the ABC comedy pilot It Takes a Village alongside Leah Remini and Cheyenne Jackson.