[4][5] In 2015, Willemse was selected in the Western Province squad for South Africa's premier high school rugby tournament, the Under-18 Craven Week.

[8] After the tournament, he was included in the South Africa Schools squad for the Under-18 International Series, involving their counterparts from Wales, France and England.

[11] He was named on the bench for their final match against England, but remained unused as the hosts completed a whitewash in the competition, winning 23–16.

[12] 2016 again saw Willemse selected for the Western Province Craven Week side for the tournament in Durban[13] and he again started all three of their matches.

In September 2019, Willemse signed for Premiership club Saracens on a 3-month deal to provide injury cover for Alex Goode and Max Malins.

[30] Damian Willemse made his international debut for South Africa in the 2018 Rugby Championship against Argentina at Kings Park Stadium in a 34–21 victory, coming off of the bench and substituting André Esterhuizen.

In response, Willemse has taken a strong interest in environmentalism and social advocacy, working to lead clean up efforts on Strand Beach and to plant over 500 trees in the local community.

[34] By partnering with the non profit, One Tree Planted and sponsors such as, Vida e Caffe and Adidas, Damian Willemse has worked to make an environmental and social impact in South Africa, while also supporting the global reforestation movement to address climate change through nature-based solutions.