Jenkner grew up in Germany and broke into Grand Prix racing as a wild card at his home German round before earning a full-time seat in 1997 aboard an Aprilia. His progress through the ranks was steady: from a 19th-place finish in 1997 to a 12th-place result in 2000 with ADAC Sachsen Honda, improving his points tally every season without yet threatening the podium.
Jenkner spent his first four full seasons on Aprilia machinery. His scoring improved each year, moving from 20 points in 1997 to 52 points in 1999, when a fifth place at Valencia represented his best result to date. He showed consistency rather than outright speed, regularly finishing inside the top ten without breaking into the front group.
Returning to Aprilia with Team UGT 3000, Jenkner finally reached the podium in 2001, collecting third-place finishes at Assen and Brno on the way to 11th overall. The 2002 season was the peak of his career: he stood on the podium five times — at Jerez, Barcelona, the Sachsenring, Estoril, and Motegi — all third places, accumulating 168 points and finishing fifth in the world championship.
The 2003 season began with three consecutive podiums for the Exalt Cycle Red Devil team: third at Suzuka, third at Phakisa, and second at Jerez. He then took his only Grand Prix victory at the Dutch TT in Assen. However, six retirements across the season — including four in a row immediately after his Assen win — proved fatal to his championship ambitions. He recovered with a third in Australia and a second in Valencia but finished sixth overall with 151 points.
Partnered with the young Marco Simoncelli at Rauch Bravo Aprilia, Jenkner added a second place at Jerez and took his first pole position at Mugello, though he could not convert it into a podium. He ended the season eighth in the standings with 122 points before graduating to the 250cc class.
Jenkner's final professional season in 250cc with the Nocable.it Race Aprilia team was a difficult transition. He failed to score in the first eight races, then found form late in the year with six straight points finishes. His best result was 12th in Japan, and he ended 23rd overall with 13 points before retiring from professional racing.
Jenkner's career exemplifies the path of a solid, durable 125cc journeyman who occasionally threatened the front of the field without ever mounting a sustained championship challenge. His 2002 campaign — five podiums and a top-five world ranking — remains his high-water mark, while his 2003 Dutch TT victory gave him a place in the record books as a Grand Prix winner.