Danny began his career as a police officer with the Newark PD. When he was a patrol officer, he testified against his training officer, Rick Peterson (Peter Greene), under a subpoena during Peterson's trial for corruption. He worked his way up to become a respected homicide detective with 87 convictions before transferring to the Honolulu Police Department. In 2001, he witnessed the murder of his partner Grace Tilwell by a pair of gangsters they had been sent to apprehend. After barely making it out alive, he notices smoke billowing from the World Trade Center and was later ordered to New York City to assist the NYPD. That day still haunts Danny and his daughter Grace was named after Tilwell. He often describes his time with the Newark PD as a "happy time" of his life as he was still happily married to Rachel and Grace was born. Danny is portrayed as a loyal cop who is married to his job first and life second. As a consequence, he is often at odds with his ex-wife Rachel for sometimes not being on time to pick up Grace. He is very loyal to his friends and colleagues, once refusing to believe that his former HPD partner Meka was corrupt even when pitted against overwhelming evidence after the partner was found dead under suspicious circumstances. Likewise, he often gives Steve the benefit of the doubt even when the latter resorts to more "unorthodox" methods in an investigation as he trusts Steve's judgment.