Costigan practices in personal and business succession law with a focus on succession planning and estate administration, trust and company structures, business structures and asset protection.

She worked with firm principal Zinta Harris — under the previous firm name of Walker Harris — from 2000 to 2003. Costigan graduated from her law degree from the University of Queensland with honours in December 2003, and was admitted to practice as a solicitor of the Supreme Court of Queensland in September 2004.

From May 2004 Kylie worked predominantly in the area of personal succession planning with a firm which specialised solely in personal estate planning, administration and litigation. She commenced working with Harris Law in May 2006.