Mellow was born on December 10, 1943 in Scranton, Pennsylvania to James and Alice (Generotti) Mellow. He is of mixed Irish and Italian ancestry. He has lived in Lackawanna County his entire life. Mellow served in the National Guard from 1962 to 1968 and, afterwards, attended Lackawanna Junior College and University of Scranton. He earned his BS in accounting from Bethel College.
He was elected to the State Senate in 1970, defeating Republican incumbent Arthur Piasecki, and was the Democratic caucus leader 1989 through 2010. Mellow represented the 22nd district, which included all of Lackawanna County and neighboring portions of Monroe County and Luzerne County. When the Democrats held the majority in the Senate from 1992 to 1994, Mellow served as the President Pro Tempore.
Mellow is a supporter of efforts to establish a medical school in Scranton. He successfully appropriated $35 million of the estimated $100 million cost in the state capital budget.
As president pro tempore, Mellow was the first lawmaker to authorize the use of television cameras on the state Senate floor to broadcast live Senate sessions.
Mellow indicated interest in running for the Governor's office in 2010. However, he ultimately did not run for Governor, and announced his intention not to seek re-election to the Senate in 2010. He was succeeded as the Democratic floor leader by Jay Costa.
Mellow was one of the leaders of the ill-fated legislative pay raise in 2005. He told constituent Bill McIntyre to "Get a life" in an email exchange about the pay raise bill. This phrase became a rallying cry for reformers that year.
Mellow is currently under criminal investigation by the FBI and IRS in relation to office leases inside buildings he once owned. Investigators raided the home in Archbald and district office in Peckville of Senator Mellow as well as searching the home of a former aide, Gabriel J. Giordano, on June 22, 2010. In July, as many as fifteen staffers were subpoenaed in connection with the investigation. In March 2012, Mellow agreed to plead guilty to charges of conspiracy to commit mail fraud and filing a false income tax return.
Mellow formally pleaded guilty on May 9, 2012 to a conspiracy charge related to using Senate staff members to do political work and filing a false tax return.
During the proceedings, U.S. District Judge Joel H. Slomsky thoroughly laid out the charges and the plea agreement for the former senator, making sure Mellow understood the plea agreement and the consequences of it. Asked if he understood that he could receive a maximum prison sentence of five years, Mellow replied “I do”. He gave the same answer when asked if he understood he was pleading guilty to a felony and that it would affect his ability to vote and hold a political office. He was sentenced to 16 months in federal prison on November 30, 2012. Mellow's state funded pension was $138,958 free of state and local income taxation. He forfeited his pension when he was sent to federal prison after his political corruption and tax evasion convictions.
After serving his sentence, in 2014 all charges were dismissed.