Rabobank GS3 (2002–2004)

From 2002 to 2004, he was in the Rabobank GS3 development team of the Rabobank cycling team. De Kort had a promising amateur career with wins in the Under 23 version of Paris–Roubaix.

Liberty Seguros–Würth (2005–2006)

In 2005 he became professional with the ProTour team Liberty Seguros–Würth of Manolo Saiz.^[4] That year he won a stage in the 2005 Tour de l'Avenir.^[5]

Astana (2007–2008)

In 2007 De Kort joined the Astana team. Following the positive tests for heterologous blood doping by team members Alexander Vinokourov and Andrey Kashechkin, Astana did not have much chance to compete in 2007 and was limited in 2008. Speaking to Dutch media, De Kort expressed his frustrations at not having the chance to compete after being in a similar situation in 2006 with the Liberty Seguros team. In 2008, De Kort finished 5th at Driedaagse van West-Vlaanderen, 12th at the Eneco Tour and 4th at the Ster Elektrotoer.

Skil–Shimano (2009–2016)

He left Astana at the end of the 2008 season, and joined the Skil–Shimano team.^[6] He stayed with the team in 2012, when it rebranded to Argos–Shimano and finished 3rd in Dwars door Vlaanderen as well as 16th in the road race at the UCI Road World Championships.^[7] From 2013 to 2016, De Kort was a vital member of the Team Giant–Alpecin squad, representing the team at 7 Grand Tours and 12 Classics.

Trek–Segafredo (2017–2021)

In 2017, De Kort joined Trek–Segafredo, and was named in the startlist for the Tour de France.^[8]

In June 2021, three of the fingers on his right hand were amputated following an accident while driving a vehicle off-road.^[9]

Grand Tour general classification results timeline

Grand Tour	2006	2007	2008	2009	2010	2011	2012	2013	2014	2015	2016	2017	2018	2019	2020	2021
Giro d'Italia	124	—	—	—	—	—	—	77	—	—	—	—	—	—	—	134
Tour de France	—	—	—	108	—	—	103	138	92	73	—	70	78	125	—	—
/ Vuelta a España	—	—	—	—	—	67	85	—	DNF	64	96	77	—	—	82	—