Harrington is the chairman and former CEO of Parsons Corporation, a technology services company in the global defense, intelligence and critical infrastructure markets. He served as chairman and CEO of the company from 2008 to 2021, following previous roles within the company, including executive vice president, chief financial officer and treasurer from 2006 to 2008; president, Commercial Technology Group, from 2003 to 2006; and president, Communications Technology Group, from 1999 to 2002, among others.

In addition to serving as chairman of Parsons, Harrington serves as a board member for J.G. Boswell Company and Cal Poly Foundation. He also served on the board of The AES Corporation from 2013 to 2020.