After graduating from North Carolina School of the Arts in 2000, he began acting professionally, and worked briefly as Mary-Louise Parker's personal assistant on several film and television projects. One of his earliest film credits was a role in David Gordon Green's George Washington (2000). In 2003, he and his college friend, Bill English, starred in The Pavement Series for Showtime, in which they played Sports inventors. In 2002, he had a role on Law & Order, and the same year booked a recurring role on The Sopranos as Meadow Soprano's (Jamie-Lynn Sigler) boyfriend Finn De Trolio. He would appear in one season of the show (fifth season), portraying a novice's perspective of the organized crime ring and most notably outing Vito Spatafore (Joe Gannascoli) one of Tony Sopranos (James Gandolfini) best earning captains. Later he would appear on Law & Order: Criminal Intent in 2004– 2008. From 2004 and on, Will appeared in various independent features including Bristol Boys a crime comedy alongside his Sopranos co-star Max Casella and David Zayas (Dexter) and a supporting role in Backseat 2005; in the same year appeared in the cult video game Grand Theft Auto: Liberty City Stories as the voice of Donald Love. Also, in that same year Janowitz joined Julianna Margulies and Michael Rispoli in Francine Volpe's Late Fragment, to be directed by Sopranos co-star Michael Imperioli. However, due to scheduling conflicts, the play did not premiere. Janowitz joined the ensemble cast of Ang Lee's Taking Woodstock in 2009 and wore a blonde wig and moustache to play Chip Monck, the festivals production designer. The same summer he went to Marietta, Georgia, to play the role of Leo Frank alongside HBO alumni Seth Gilliam (The Wire) in The People v. Leo Frank. The film explores the life and trial of Leo Frank an American born Jew who was lynched in Marietta, Georgia, on August 17, 1915, after being wrongly accused of the murder of a 13-year-old girl who worked in his pencil factory. The case is widely regarded as a miscarriage of justice.

In the midst of his acting, Janowitz has had several plays of his own creation produced. The most notable; Gnome a character he created at North Carolina, then more extensively at Upright Citizens Brigade Theatre. The show premiered at New York City's Zipper Theater 2006. The two-man play explores the life of Gnome; a reclusive hundred and fifty-year-old author who claims to be the offspring of a Jewish mother and forest dwelling creature who left New York City and has taken up permanent residence in Central Park. Additionally, Janowitz wrote and developed a television series for Fox Studios called SuperEgo 2009 in which he plays Charlie Knight, a young, Psychiatrist who uses an unorthodox approach to unlock patients potential.