In March 1962, Pat Sullivan, while a senior in high school, began his career with the Littleton (CO) Police Department as a police officer/dispatcher. In February of 1966, he began an active duty commitment with the U.S. Army, which was completed in July of 1969. He returned to Littleton and resumed his career with the police department, where he worked his way up obtaining the rank of lieutenant in charge of the patrol division. In August of 1977, he received a Bachelor of Science Degree in Law Enforcement Administration from Metropolitan State College in Denver and also graduated from the 111^th session of the FBI National Academy.

In January of 1979, Pat accepted the position of Patrol Captain with the Arapahoe County (CO) Sheriff’s Office. He later directed the criminal investigation division before being appointed undersheriff in 1983. With the unfortunate death of the previous sheriff, Pat was appointed Sheriff of Arapahoe County in June of 1983. He has subsequently been reelected in the last five elections.

In July of 1988, the Arapahoe County Sheriff's Office became the first Colorado sheriff's office to be nationally accredited by CALEA. In June of 1993, the agency was one of only nine sheriff's offices to receive the prestigious Triple Crown Award recognizing simultaneous accreditation by CALEA, the Commission on Accreditation for Corrections, and the National Commission on Correctional Health Care. Pat became a CALEA Commissioner in 1994 and currently chairs the Standards Review and Interpretations Committee. The Arapahoe County Sheriff's Office was one of the co-hosts of CALEA’s March 1999 Commission meeting in Denver.

Sheriff Sullivan is extremely involved in professional and community activities. He is a member of the President’s National Commission on Crime Prevention and Control; a member of the Executive Committee and Board of Directors, and Chairman of the Law and Legislative Committee of the National Sheriffs’ Association; and a member of the National Law Enforcement and Correction Technology advisory board.