DeMentri graduated from Temple University with a B.A. in broadcast journalism. He also played football for the Temple Owls as a linebacker from 1983 to 1985. He began his career as a Sports Producer for WPVI-TV in Philadelphia and worked for WOI-TV as a Weekend Anchor in 1989. He was later an Investigative Reporter and Anchor for WDIV-TV in Detroit, Michigan and WPRI-TV in Providence, Rhode Island.
In 1993 DeMentri joined CBS' flagship WCBS-TV in New York as a reporter, and became anchor of the station's weekend evening newscasts. He stayed there until 2003, when he moved to NBC's Philadelphia affiliate, WCAU-TV. DeMentri won several awards for his reporting for WCBS and WCAU, including seven Emmys for investigative reporting and a National Edward R. Murrow Award for his reporting on the September 11, 2001 attacks on the World Trade Center. While at WCAU DeMentri served as anchor for the early evening newscasts as well as ones produced for WPHL-TV by the station.