David Koehler was born and raised in South Dakota. He received his Bachelor of Arts degree from Yankton College, in Yankton, South Dakota in 1971. Three years later, he received a Masters of Divinity from United Theological Seminary in Dayton, Ohio. From 1972 to 1978, was a staff member of the National Farm Worker Ministry (NFWM), working on behalf of migrant farm workers. Upon moving to Peoria in 1978, became a community organizer, and later, the program manager for Peoria Friendship House.

In 1985 was hired as the first Executive Director of PALM, the Peoria Area Labor Management Council, a not-for-profit organization founded to foster cooperation and participation between labor unions and management for both public and private sector groups. In 1992, also became the President for Labor Management Cooperative Health Programs, Inc, a wholly owned subsidiary of the PALM. LMC Health Programs is a purchasing cooperative organization dedicated to improving the quality and value of health care for its members and the community.

From 1982 to 1988, served on the Peoria County Board as a Democrat from District 5, and from 1989 to 1997, served on the Peoria City Council (non-partisan) from District 3, serving as Mayor Pro Tem from 1993 to 94. He also served on many boards and commissions with subjects ranging from labor-management relations to health care. He was elected to the Illinois Senate in 2006 and promoted to Majority Caucus Whip in 2018 and Assistant Majority Leader in 2019. He is married with three daughters, two sons-in-law, and one daughter-in-law. He is also the happy grandfather of three granddaughters and one grandson.