Costello has over 30 years of prosecution experience, working in various counties in the Twin Cities area. He has tried hundreds of jury trials and court trials, and he has handled thousands of criminal, traffic, and ordinance violation cases. He also handles arbitrations involving labor and employment law issues, representing Teamsters Local No. 120 in St. Paul, Minnesota. Costello is also counsel to the International Brotherhood of Teamsters General Secretary-Treasurer C. Thomas Keegel.