He previously served as an associate judge of the Boston Municipal Court.

In 2014, Governor Deval Patrick nominated Joun to serve as an associate judge of the Boston Municipal Court.

He has been nominated to a seat vacated by Judge George A. O'Toole Jr., who assumed senior status on January 1, 2018.

[9] On November 15, 2022, a hearing on his nomination was held before the Senate Judiciary Committee.

[11] On January 3, 2023, his nomination was returned to the President under Rule XXXI, Paragraph 6 of the United States Senate.