Kilmartin was born February 18, 1962 in Rhode Island. Kilmartin attended William E. Tolman High School in Pawtucket and graduated in 1980. Kilmartin was sworn in as a police officer with the Pawtucket Police Department in June 1984 while continuing his studies at Roger Williams University, where he earned his Bachelor of Science in 1988. He later returned to school, and earned his Juris Doctor in 1998 from Roger Williams University School of Law. Kilmartin eventually rose through the ranks of the Pawtucket Police Department, ultimately attaining the rank of captain of the Department's prosecution division. He retired from the police department in 2008, after 24 years of service.
Peter Kilmartin was first elected as a state representative on November 6, 1990 into the 80th district. Upon downsizing of the House of Representatives from 100 to 75 members in the 2002 election, Kilmartin ran unopposed and was elected to serve in the 61st district. Running unopposed, Kilmartin was most recently re-elected on 4 November 2008. He served as Majority Whip from 2005–2010, and during the 2009-2010 legislative session, Kilmartin served on the House Committee on Corporations, House Committee on Labor, House Committee on Rules, and the Joint Committee on Highway Safety .
Kilmartin announced he would not seek an eleventh term in the House of Representatives at the end of the 2010 session, and on February 23, 2010 he announced his intention to run for the office of Attorney General of Rhode Island. He was elected as Rhode Island's Attorney General on November 2, 2010.