[3][4] Kienle was exposed to the sport of triathlon for the first time at the age of 8 and then knew that he wanted to grow up to be a professional triathlete.

Kienle competed in his first XTERRA Triathlon in 2005 in Germany, in which he won and had "an absolutely great race."

[6] In 2009, he won Ironman 70.3 Germany in Wiesbaden, holding off 70.3 World Champion Michael Raelert.

[8][9] He placed 4th a month later at the 2012 Ironman World Championships, posting the second fastest bike split of the day - despite incurring a flat tire.

[10] The following year, in 2013, he posted somewhat lackluster results during the season;[11] however, Kienle defended his Ironman 70.3 title by winning the 2013 World Championship race.