Lietz began competitive motorsport in karting in 1998 and 1999 before moving into single-seater racing. He contested Formula BMW ADAC from 2000, finishing sixth in 2001 with one win and four podiums. He then competed in German Formula Three in 2002 and the Formula Three Euro Series in 2003. He also won the Austrian Rally Challenge in 2001, a title that reflected an early breadth of driving ability.
From 2004, Lietz committed to Porsche machinery. He contested the Porsche Carrera Cup Germany in 2004 and 2005 and the Porsche Michelin Supercup from 2004 through 2006, finishing third in the Supercup standings in 2006 with two race wins.
Porsche signed Lietz as a factory works driver in 2007. He immediately justified that status by winning the overall International GT Open championship that year, sharing the car with Swiss driver Joël Camathias and scoring four victories. He also won the Le Mans 24 Hours in the GT2 class in 2007 and finished second at the Spa 24 Hours.
In 2008, Lietz was competitive across the International GT Open — five wins and a second-place championship finish — as well as in the Le Mans Series and the 24 Hours of Le Mans, where he took pole position in GT2.
Lietz won the GT2 class title in the Le Mans Series in both 2009 and 2010, on each occasion sharing the car with Marc Lieb. The 2010 title was accompanied by a class win at the 24 Hours of Le Mans — his second overall Le Mans class victory. He finished second in the International GT Open in both seasons as well.
In 2011, Lietz won the Nürburgring 24 Hours in the GTH Hybrid class with Manthey and also won at Laguna Seca in the American Le Mans Series. The following year he won the 24 Hours of Daytona in the GT class, sharing with Andy Lally, Rene Rast, and John Potter at Magnus Racing — his first Daytona 24 victory. He added wins at Spa and Fuji in the FIA World Endurance Championship's GTE Pro class in 2012.
In 2013, Lietz took his third Le Mans 24 Hours class win, this time in the GTE Pro category driving for Porsche AG Team Manthey.
In 2014, Lietz won the 24 Hours of Daytona a second time in the GTLM class. At Le Mans he finished third in GTE Pro with Manthey, sharing with Frédéric Makowiecki and Marco Holzer. The season placed him third in the WEC GT drivers' standings.
The following year, 2015, delivered the peak achievement of Lietz's career. He won the FIA World Endurance Championship for GT Drivers in the GTE Pro class, claiming the title with victories at the Nürburgring and Austin WEC rounds among other strong results. He was also part of the winning Porsche trio when the manufacturer achieved its first overall victory at the American Petit Le Mans in October 2015.
In 2018, Lietz won the Nürburgring 24 Hours SP9 class with Manthey Racing, sharing with Frédéric Makowiecki, Patrick Pilet, and Nick Tandy.
Lietz continued in the WEC through the GTE Pro era. His 2022 Le Mans result produced a fourth Le Mans class win, this time in LMGTE Pro with the Porsche GT Team. As GT3-based machinery replaced purpose-built GTE cars in the WEC's GT class from 2024, Lietz adapted successfully to the LMGT3 category. He won the Le Mans 24 Hours LMGT3 class in 2024 with Manthey EMA and again in 2025 with Manthey 1st Phorm. The 2025 season also brought the FIA Endurance Trophy for LMGT3 Drivers, making him a two-time World Endurance Champion across different GT categories.
In 2026, Lietz continues with Manthey 1st Phorm in the WEC LMGT3 class and with Proton Competition in the European Le Mans Series LMGT3 class.
Across more than 90 WEC starts, Lietz has recorded 14 wins and 9 pole positions in the championship. His total career figures, per the DriverDB database, include 49 race wins and 112 podiums. The FIA classifies him at the Platinum level — the top tier for professional GT drivers. His sustained record of success across GT2, GTE Pro, GTLM, and LMGT3 categories over nearly two decades marks him as one of the most accomplished endurance GT drivers in the sport's history.