During his early years, Jansen used to open the batting. At the age of nine, in a 20-over match, he scored 164 not out. His father watched the match and identified his son's talent. He trained Jansen in the nets along with his twin brother Duan.[1]Duan also plays cricket for North West.[2]
In January 2021, Jansen was added to South Africa's Test squad for their series against Pakistan.[14]
In May 2021, Jansen was named in South Africa's Test squad for their series against the West Indies.[15] In December 2021, Jansen received another call-up to South Africa's Test squad, this time for their home series against India.[16] He made his Test debut on 26 December 2021, against India.[17] His maiden test wicket was Jasprit Bumrah, caught at third slip by Wiaan Mulder.[18]
In January 2022, Jansen got his maiden One Day International (ODI) call-up, for South Africa's home series against India.[19] He made his ODI debut on 19 January 2022, for South Africa against India.[20] In May 2022, Jansen was named in South Africa's Twenty20 International (T20I) squad for their away series against India.[21] He made his T20I debut on 17 June 2022, for South Africa against India.[22] He also played in the 2023 Cricket World Cup in India.[23]
In May 2024, he was named in South Africa’s squad for the 2024 ICC Men's T20 World Cup tournament, where South Africa finished runners up.[24]
In November 2024, he was named in South Africa's squad for the Twenty20 International series against India.[25] In the third match, he scored 54 off 17 deliveries, setting a record for the fastest T20I half-century against India.[26]
In November 2024, he was named in South Africa's squad for the Test Series against Sri Lanka.[27] On the second day of the 1st Test Series, he bowled his best figure of 7/13,[28] toppling Sri Lanka to a score of 42, their lowest score in Test cricket.[29] This feat made him the first ever South African to take a 10-fer at Kingsmead, Durban.[30] He is also the first left-arm South African seamer to gain the feat.[31]
Andile Phehlukwayo and Lizaad Williams were not initially in the squad, but were named as replacements for Anrich Nortje and Sisanda Magala in the final squad.