There is growing recognition that people with disability have the right to be involved in making decisions that affect their lives. Decision-making support has emerged as one way to support people with cognitive disability to make decisions, however, there is a paucity of research that explores how disability support workers can be upskilled to provide decision-making support to this group. The aim of the research was to explore the impact of an evidence-based online training course on disability support workers of adults with cognitive disability. Changes in knowledge about decision-making support and confidence in providing decision-making support were explored, attitudes towards decision-making support, and facilitators and barriers. Participants completed an online training course and responded to a survey on three occasions: baseline, post-training, and at 2-month follow-up. Ninety-nine disability support workers across Australia participated in the online training and completed the baseline and post-training surveys. Thirty-six participants completed the training and all three surveys. The results revealed that there were statistically significant improvements in knowledge, confidence, and attitudes from baseline to post-training, which were maintained at 2-month follow-up. Barriers to decision-making support included service providers or other supporters, including the family of the person with cognitive disability, whilst a key enabler was knowing about the decision-making support principles. This research demonstrates that an evidence-based online training course about decision-making support can be effective in building capacity in disability support workers. There are, however, several barriers that must be addressed to facilitate the implementation of decision-making support.