[8] Marx made his senior debut for the Golden Lions in the 2014 Vodacom Cup by starting in their 18–16 opening day victory over the Leopards in Potchefstroom.

[14] In 2016, Marx was included in a South Africa 'A' squad that played a two-match series against a touring England Saxons team.

Marx made his debut for the Springboks on 17 September 2016, the same date as team mate Francois Louw's 50th test.

Marx replaced Springbok captain Adriaan Strauss in the 43rd minute, but had a disappointing debut, with South Africa losing to New Zealand 13-41.

Marx earned his first start for the Springboks on 5 November 2016, putting a 51-minute performance into a 31-31 draw against the Barbarians Club, prior to being replaced by Bongi Mbonambi.

The performances of Marx, as well as fellow forwards Steven Kitshoff and newly-appointed Springbok Captain Eben Etzebeth allowed South Africa to compete well against the All Blacks, with Marx making four turnovers and many tackles on defence, also setting up replacement loose forward Jean-Luc du Preez for a try and scoring the final try of the game.

Marx's try was converted by Lions teammate Elton Jantjies to make the final score a narrow 24-25 loss to the All Blacks.

[22] Marx was part of South Africa's squad that won the 2023 Rugby World Cup; however, he missed most of the tournament due to a knee injury suffered in training.