The tribunal found the relationship had taken place between June and August 2010, while he was a partner in the law firm Adams and Horsley. Mr Horsley first met the woman in April 2008, when he acted for her in the Tauranga Youth Court when she was 16 years old.
He subsequently acted for her on a number of criminal matters over two- and-a-half years.
Mr Horsley told the tribunal the girl wanted him to represent her because of her "trust and confidence" in him.
In relation to the second charge of misconduct, he faced allegations of "disgraceful and dishonourable" conduct and misleading the law standards committee.
Mr Horsley admitted lying to the committee by first denying the relationship with the young woman existed and also lied about when their relationship began. "I didn't say at the time that I was in a relationship with her," he told the tribunal.
"But I did indicate that I was there on a two-fold basis, it was not only as her lawyer but as a person who was trying to support her and cared about her."
A penalty hearing in relation to the charges is yet to be scheduled.
