[4] Siemann made his international debut for the United States at the 2011 IPC Athletics World Championships.

[6] He again represented the United States at the 2016 Summer Paralympics, and finished in fourth place in the 800 metres T53 event, with a time of 1:41.11, two-one hundredths of a second behind bronze medalist Brent Lakatos.

[8][9] He represented the United States at the 2017 World Para Athletics Championships in the 100 metres T53 event.

[10] He again represented the United States at the 2019 World Para Athletics Championships and finished in fourth place in the 800 metres T53 event with a time of 1:41.63.

At six days old, a doctor left Siemann's umbilical catheter open, which caused him to lose a third of the blood, resulting in spinal shock and paralyzed him from the waist down.