Mr. Diegnan was admitted to the New Jersey bar in 1974, and has a private law practice in South Plainfield.

A former instructor of law and accounting at Middlesex County College, Mr. Diegnan has sponsored numerous bills supporting education including the New Jersey Student Tuition Assistance Rewards Scholarship (NJSTARS) program, which helps eligible students with county and four-year public college tuition. He sponsored tenure reform legislation that protects teacher bargaining rights while ensuring accountability in the classroom. He also sponsored legislation protecting student athletes by requiring school districts to develop policies to address concussions and to detect risk factors for cardiomyopathy and cardiac arrest.

Mr. Diegnan joined the Senate on May 9, 2017, filling a vacancy. He won re-election that November. He previously served eight terms in the Assembly. The senator is married to Anita Carchia. They have two daughters, Heather and Tara, and four grandchildren- Vanessa, Alexa, Brooke and Andrew.