She studied at the Amsterdamse Hogeschool voor de Kunsten.
She had parts in The Preacher, Full Moon Party and Deuce Bigalow: European Gigolo and presented Idols. She also played Belle in the Dutch Musical version of Beauty and the Beast and Jane in the Dutch Musical version of Tarzan. She also sang a song against cancer, called "Vecht Mee" (English: Fight With Us) with Dutch rapper Yes-R. From late 2011, she stars as Glinda in the Dutch premiere production of Wicked.
She was taught in classical-, modern-, jazzand tapdance, acting, singing and musical repertoire.
She played in several musicals, including Crazy Foreign You, Kunt u mij de weg naar Hamelen vertellen, mijnheer?, Saturday Night Fever, 42nd Street, Beauty and the Beast, Tarzan and the current production of Petticoat. She played several guest roles in television shows like Baantjer, Intensive Care, De Band, Meiden van De Wit and Kinderen Geen Bezwaar.
She also hosted Staatsloterij Live and the Dutch version of Popidol, Idols. Janzen played several parts in movies:De Dominee, Feestje, Deuce Bigalow: European Gigolo, Volle Maan, Alles is Liefde, Kicks and the made-for-television film Loverboy.
In December 2002 her single Achter De Sterren, the title song from the movie Science Fiction was released.
She got nominated for Best female lead in 2006 as Belle in Beauty and the Beast and in 2007 she got nominated for the same prize for her role as Jane in the musical Tarzan. Because of her pregnancy she had to temporarily leave her part in the musical Tarzan from September 2008 through April 2009. She was replaced by Bente van den Brand.
After Tarzan she played a part in the Disney Musical Sing-Along.
In 2010 Janzen can be seen in the musical Petticoat. lieutenant"s an original Dutch musical, written especially for her.
After her role in Petticoat she will go on to play in the Dutch rendition of the Broadway musical Wicked. In 2011 Janzen ended her contract with the Dutch television channel AVRO and signed a contract with RTL. According to her, hosting the Award shows she had been hosting at the AVRO for the fourth year in a row, would make her performance look cheap.
She was enthused by the idea of experiencing other things.
Janzen has been in a relationship with Marco Geeratz for a few years now, who brought three kids from an earlier relationship into their relationship. On December 15, 2014 the couple got married in London.