Pat O'Brien was born on October 8, 1969 in Sioux City, Iowa. He is the youngest of four children born to Mike and Gladys O'Brien. His older siblings are Dan, Jim and Joan. Pat has never been married and has no children. He has lived in Jacksonville since 1972 and has attended St. Jude's Catholic Church since then. Mike O'Brien, Pat's father, opened a McDonald's franchise in Jacksonville on January 3, 1973. His father died in 1990 and Jim O'Brien took over the family business.

Pat attended public schools and graduated from Jacksonville High School in 1988. In high school, he worked as a string reporter for the local paper (now the Jacksonville Patriot) and as the editor of the Jacksonville High School newspaper. Pat also worked at the family's McDonald's restaurant as a cook (meat rotation technician) fulltime in high school and college (during the summer).

Pat graduated from the University of Arkansas at Fayetteville with a Bachelors of Arts in 1992 with a major in political science and a minor in history. Pat graduated from the University of Arkansas at Fayetteville School of Law with a Juris Doctorate in 1995. Pat became a member of the Arkansas bar in August, 1995.

Pat maintained a general practice law from 1995 to 2000. His first law partner, Mike Wilson, was serving in the Arkansas House of Representatives at the time. In 1996, Mike Wilson left for the Worker's Compensation Commission and Pat became partners with Will Bond. Pat handled all kinds of cases such as estates and trusts, criminal defense, personal injury, contracts and general litigation, though he never handled any divorces. Pat is still a member of the Arkansas Trial Lawyers Association and a member of the Arkansas Bar Association in good standing though he does not actively practice law. Pat never received any ethical complaints, malpractice claims or investigations during the time he has practiced law.

Pat was on the Jacksonville Chamber of Commerce Board of Directors for a term and served as President of Downtown Jacksonville, Inc. DJI was dedicated to revitalizing the downtown area of Jacksonville and attracting new businesses.

In September, 1999, Pat was elected to Zone 6 of the Pulaski County School Board. An opponent filed against Pat but then withdrew from the race after realizing he did not live in Zone 6. Pat served on the PCSSD board for one term from 1999-2003. Pat did not run for a second term to the board. Pat has been a strong advocate for more local control of the Jacksonville area schools.

In April, 2000, Pat left the practice of law to become Director of Operations for JKP Foods, Inc. This meant Pat was in charge of running the daily operation of seven McDonald's restaurants in North Pulaski County (Jacksonville, Sherwood and North Little Rock). Pat took the job because his brother, Jim, needed help running the business following his divorce. Pat was responsible for leading a staff of 350 employees doing sales of over $10 million per year.

On January 1, 2003, Pat became Chief of Staff for the 22nd Judicial District's Prosecuting Attorney's Office. The office was led by Robert Herzfeld. Pat was in charge of administrative staff, litigating complex cases and cleaning up delinquent case files that had not been prosecuted.

In 2004, Pat campaigned for and was elected to the office of Pulaski Circuit/County Clerk. He won the Democratic nomination in a run-off against Larry Crane and then the general election against Chief Deputy Circuit/County Clerk, Janice Hay. Pat took over the troubled office on January 1, 2005, cleaned it up and was re-elected to the office in 2006 and 2008 without opposition.

Pat is an avid Razorback fan and enjoys sports in general. He plays basketball and golf, and occasionally duck hunts.