He studied for the Priesthood at Christ the King Seminary in Olean, New York where he earned his Master of Divinity Degree. He was ordained to the Priesthood by Most Reverend Bishop Bernard Casey, D.D. at the Cathedral of Saint John the Baptist in Paterson, NJ in May 1969. His first assignment was Associate Pastor of Our Lady of Lourdes Parish in Paterson, NJ from July 1969 to June 1973. It was in 1969 that he formed the Christian Riders Motorcycle Club.
He was assigned to St. Gerard Majella Parish in Paterson as Associate Pastor from June 1973 to January 1975 and as Administrator/Co-Pastor from January 1975 to October 1987. In November 1987 he became Rector of the Cathedral of Saint John the Baptist. He also served as Administrator at Saint Agnes Church, Paterson, NJ from July 2002 to June 2007.
In 1973 he was Member of the Priests' Senate, from June 1984 to December 1990, he was Dean of the Paterson Deanery (Passaic County I) for three terms, July 2005 to April 2011. He was named to the Office of Honorary Prelate in April 1989.
Reverend Monsignor Mark J. Giordani served as Chaplain for the Boy Scouts and Chaplain for the Haledon, NJ Police and Fire Departments for many years.
He also served as a U.S. Army Reserve Chaplain in Lodi, New Jersey from 1977-1980, Chaplain to the Paterson Police Department since 1977, Chaplain to the Passaic County Sheriff's Department, Chaplain to the Passaic County Jail, Chaplain to the Constables Association, Chaplain to the Port Authority of New York and New Jersey, Chaplain to the Passaic County Police Academy and Chaplain to the Fidelians in North Haledon.