A native of Butte, Montana, McGrath graduated from the University of Montana with a BA in business administration in 1970 and received his JD from Gonzaga University Law School in 1975. He was a Reginald Heber Smith Community Lawyer Fellow in Reno, Nevada from 1975 to 1976, and Montana's Assistant Attorney General from 1976 to 1982. He served as County Attorney for Lewis and Clark County from 1982 to 2000.
McGrath served five terms as Lewis and Clark County Attorney. In his 18 years as a prosecutor, McGrath focused on family violence issues, including domestic abuse and sexual assault of children.
He is a former chair of the Conference of Western Attorneys General (CWAG) and served as president of the Montana County Attorneys' Association.
McGrath is a candidate in the 2008 election for the position of Chief Justice of the Montana Supreme Court.
He and his wife Joy have two sons, Pat and Chris, and one granddaughter, Alexis.