Owned his first club as a two year old, when he would play with a little putter, but really started playing at the age of six.

His parents ran a golf club restaurant at Burge Overbach in Cologne.

Turned professional in 2000 and first earned playing rights on Tour after coming through Qualifying School in 2002.

Earned his first victory at the 2004 Dunhill Championship as he defeated the French pair of Grégory Havret and Raphael Jacquelin in a play-off. Had to wait eight years before his second DP World Tour win at the Alstom Open de France. In the intervening years, he partnered compatriot Bernhard Langer to victory at the 2006 WGC-World Cup

Won his third DP World Tour title at the 2013 Trophée Hassan II and followed that with the biggest victory of his career to date at the 2014 BMW Masters in China, defeating Ross Fisher and Alex Levy in a play-off, propelling him to a career-high seventh place finish in the Race to Dubai.

Returned to the Challenge Tour in 2021 after losing European Tour playing rights. Captured the hearts of the golfing world as he played 13 tournaments in a row before triumphing at the Le Vaudreuil Golf Challenge.

His victory in France earned him the final spot at The 149th Open at Royal St George's the following week, where he quickly became a fan favourite. Siem carded a three under par 67 on his 41st birthday to sit T9 after the first round. He remained inside the top ten after the third round before eventually finishing in a share of 15th.

Finished 15th on the 2021 Road to Mallorca Rankings to earn a return to the DP World Tour the following year.

Made his 500th DP World Tour appearance at the 2023 Singapore Classic and two weeks later returned to the winner's circle as he triumphed at the Hero Indian Open, more than eight years after his last win in China.

Claimed his sixth DP World Tour title at the 2024 Italian Open presented by Regione Emilia-Romagna, maintain his 100 per cent success rate in play-offs as he defeated Tom McKibbin on the first sudden death hole.