Abstract Background Low functional capacity, malnutrition, and anaemia are associated with an increased risk of complications after surgery. These high-risk indicators can be improved through preoperative interventions. The aim of the study was to examine the effect of screening for modifiable high-risk factors combined with targeted interventions on postoperative complications in patients undergoing colorectal cancer surgery. Methods A controlled before-and-after study was conducted including patients with colorectal cancer undergoing elective curative surgery between August 2015 and October 2018, in two institutions (intervention and control hospital). The intervention consisted of a screening for anaemia, low functional capacity, and nutritional status and their implementation (iron supplementation, prehabilitation, nutritional supplements, and consultation with a dietician), for a minimum of 4 weeks before surgery. The primary outcome was a composite measure consisting of unplanned admission to the intensive care unit, complications with Clavien–Dindo score of 3a or above, length of hospital stay less than 10 days, readmission, or death within 30 days during the postoperative course. Results A total of 1591 patients were included for analysis with 839 at the intervention hospital and 752 at the control hospital. In a difference-in-difference analysis, adjusted for age, sex, smoking, stage of disease, ASA score, surgical approach, and surgical procedure, the intervention was associated with a 10.9 per cent (95 per cent c.i. 2.1 to 19.7 per cent) absolute risk reduction of a complicated postoperative course, primarily due to a reduction in severe complications. Conclusion The combined intervention of screening and prehabilitation was associated with a decreased risk of a complicated course, primarily in a reduction of severe complications.