A Democrat, he served in the Iowa Senate from 2003 until 2016.

He received a DVM from Iowa State University and worked as a veterinarian in Davenport, running a veterinary clinic for low income families.

As of October 2011[update], Seng served on several committees in the Iowa Senate - the Commerce, Ethics, and Natural Resources and Environment committees.

He also served as chair of the Agriculture Committee and as vice chair of the Ways and Means Committee, as well as serving as a member of the Economic Development Appropriations Subcommittee.

Seng died in office on September 16, 2016, from brain cancer.