Assistive robotics can improve the lives of people with mobility impairments, but widespread acceptance depends on understanding how trust forms during human-robot interaction (HRI). This study reports on a real-world mobile navigation experiment involving 27 participants who used an autonomous wheelchair to deliver packages to predetermined locations. The performance of the wheelchair was manipulated to create good and bad performance conditions. The participants’ trust in the robot was measured using established trust questionnaires before and after the interaction. The results indicate that exposure to the robot and its performance significantly impact participants’ self-reported levels of trust in the robot. Specifically, scores on trust factors relating to the robot taken after the experiment were significantly higher than those taken before, suggesting that exposure to the robot is an essential element of the trust-building process. The study also found that poor robot performance during the interaction negatively affected participants’ perception of the robot’s behaviour and their attitudes towards it. The study concludes with implications for the design and development of autonomous assistive devices, highlighting the importance of potential users’ exposure to assistive robots for building trust in the system and the negative impact of bad robot performance on people’s attitudes towards the technology.