He attended Sky View High School in Smithfield, Utah, where he was a three-time All-American at the Foot Locker Cross Country Championships.

[4] Mantz also represented the United States in the junior race at the 2015 IAAF World Cross Country Championships in Guiyang, China.

[5] Mantz committed to Brigham Young University (BYU), among offers from several other institutions, including Princeton and Furman.

[6] After high school, Mantz took a two-year hiatus from running to serve as a missionary for the Church of Jesus Christ of Latter-day Saints in Ghana.

He placed fifth in the 5000 m and eighth in the 10000 m.[14] ln December 2021, Mantz won the USA Half Marathon Championships in Hardeeville, South Carolina, with a time of 1:00:55.

Running with a large group of leaders, including Eliud Kipchoge, Evans Chebet, CJ Albertson, Mantz reached halfway in 1:02:20.

[18] At the 2023 Chicago Marathon, Mantz was the sixth place finisher and top American, lowering his personal best by 29 seconds to 2:07:47.

Although Mantz's personal best was the 56th fastest in the field, he produced an eighth place finish in the marathon at the 2024 Paris Olympics.

While recovering from the injury, he swam up to 10,000 yards in one day and worked on the hills he would face in Paris by running uphill and getting driven downhill in repetition.