In December 2018, he launched to the International Space Station, as Flight Engineer on Expeditions 57, 58 and 59; he returned to Earth on 25 June 2019.

[4] On 19 September 2011, NASA announced that Saint-Jacques would serve as an aquanaut aboard the Aquarius underwater laboratory during the NEEMO 15 undersea exploration mission from 17–30 October 2011.

[7] In 2013, Saint-Jacques served as cavenaut into the ESA CAVES[9] training in Sardinia, alongside Soichi Noguchi, Andreas Mogensen, Nikolai Tikhonov, Andrew Feustel and Michael Fincke.

[10] On 3 December 2018, Saint-Jacques launched to the ISS on board Soyuz MS-11, alongside commander Oleg Kononenko and fellow flight engineer Anne McClain.

On 8 April 2019, Saint-Jacques conducted his first spacewalk,[12] becoming the fourth Canadian astronaut to take part in an EVA and the first to do so in 12 years.

[4] Saint-Jacques is affiliated with the Collège des médecins du Québec, the College of Family Physicians of Canada, Ordre des ingénieurs du Québec, International Society for Optical Engineering, and the Cambridge Philosophical Society as a Life Fellow.

[4] Saint-Jacques began his career in 1996 as a biomedical engineer at the Quebec firm Electromed with secondment to Lariboisière Hospital, working on the design of radiological equipment for angiography and image analysis algorithms of cineangiography.

[4][17] Prior to joining the Canadian Space Program, Saint-Jacques was a medical doctor and the Co-chief of Medicine at Inuulitsivik Health Centre in Puvirnituq, Quebec since 2007.