Before his service as Clerk, McDonell served on the Dane County Board of Supervisors from 1996 to 2013, including eight years as chair.

He also organized other clerks to do so, after a federal court struck down the state's ban in Wolf v. Walker, but before the ruling was put on hold pending an appeal.

His early career also included employment in the state Department of Administration and public service on the boards of the Tenant Resource Center and the Madison Community Cooperative.

Highlights of his service include adoption of a Regional Transit Authority (which was later repealed by Republicans[4] when they took over the Wisconsin Legislature) and creating the first county domestic partner registry.

Although he was permitted to appoint himself for a longer term as acting County Executive, he declined, saying that it would be 'undemocratic' and giving him an unfair advantage in the upcoming election to succeed Falk.

[7] Peters declined to run for a full term, and McDonell entered the race to serve as the next County Clerk.

In August 2012 he won a 4-way primary for the Democratic nomination[8] and was unopposed in the November general election and took office in January 2013.

[11] A primary duty of the Dane County Clerk's Office, in addition to overseeing elections, is to issue marriage licenses.

He had prepared for that possibility in advance,[12][13][14] having staff, judges and court commissioners at the ready for a “marriage rush” that day and adding open office hours into the following weekend.

[21][22] McDonell helped organize communication among clerks on how to approach these new laws, which made Wisconsin voting requirements among the most restrictive in the nation.

This resulted in legal challenges[23] and rapidly changing guidance from courts and elected officials, while requests for absentee ballots surged.

[33][34] In addition to improving the physical security of the Clerk’s office, he has also administered training courses for election works in de-escalating conflict at the polls.