what did he start his career as?
He was a Broadway actor.
Jonathan Groff
Groff earned his Actors' Equity Association card in 2005, with the musical "Fame" at the North Shore Music Theatre. That same year, he made his Broadway debut as an understudy for the lead role and a swing for the musical "In My Life" by Joseph Brooks. Groff originated the role of Melchior Gabor in the Broadway production of the rock musical "Spring Awakening". 
who does the voice of kristoff from frozen Jonathan Drew Groff who is he? American actor and singer where is he from? Lancaster, Pennsylvania
wiki:10840458
Jonathan Groff [SEP] 2005–2009: Career beginnings and "Spring Awakening"
Groff earned his Actors' Equity Association card in 2005, with the musical "Fame" at the North Shore Music Theatre. That same year, he made his Broadway debut as an understudy for the lead role and a swing for the musical "In My Life" by Joseph Brooks. Groff originated the role of Melchior Gabor in the Broadway production of the rock musical "Spring Awakening". He played the role from its debut on December 10, 2006, through May 18, 2008, when he departed the production with his co-star and best friend Lea Michele. He had previously played the role in the original Off-Broadway production earlier in the summer of 2006.