On July 29, 2022, President Joe Biden announced his intent to nominate Joun to serve as a United States district judge of the United States District Court for the District of Massachusetts. On August 1, 2022, his nomination was sent to the Senate. He has been nominated to a seat vacated by Judge George A. O'Toole Jr., who assumed senior status on January 1, 2018.[8] On November 15, 2022, a hearing on his nomination was held before the Senate Judiciary Committee.[9] On December 8, 2022, his nomination was reported out of committee by a 12–10 vote.[10] On January 3, 2023, his nomination was returned to the President under Rule XXXI, Paragraph 6 of the United States Senate. He was renominated on January 23, 2023.[11] On February 9, 2023, his nomination was reported out of committee by a 12–9 vote.[12] On July 12, 2023, the Senate invoked cloture on his nomination by a 51–46 vote.[13] Later that day, his nomination was confirmed by a 52–46 vote.[14] He received his judicial commission on July 14, 2023.[15] He became the first Asian American man on the federal bench in Massachusetts.[16]