Insufficient physical activity is widespread amongst older office workers and has adverse effects on their physical fitness and psychological wellbeing, as well as being a cause of pain and of pain intolerance. Health challenges in older office workers may have an impact on their work productivity, quality of life, and involvement in the workplace. Differing exercise interventions have been employed to improve older workers’ health conditions, with Tai Chi being reported to be a suitable physical activity intervention for this purpose. Tai Chi appears to improve older adults’ physical function and psychological wellbeing, and to reduce pain in older adults. Additionally, many studies have found the benefits of resistance training in the workplace for older adults, especially for work-related pain issues. However, a systematic review found that few studies have examined the use of combined Tai Chi and resistance training to improve the physical function of older adults, and there is a lack of studies regarding the effects of this combined programme on psychological wellbeing and pain. Further to this, no study has examined the use of Tai Chi combined with resistance training in older office workers. This study aimed to investigate the effectiveness of a 12-week Tai Chi combined with resistance training programme in improving physical fitness and psychological wellbeing, and in reducing pain in older office workers. This programme was compared to a Tai Chi exercise only programme in a group of 40 older office workers (i.e., ≥ 55 years) who engaged in less than 60 minutes of accumulated moderate intensity physical activity per week and were employed by a university in Australia. A pilot randomised controlled trial (RCT) design was applied in this study. The first 10 movements of the 24-step simplified Tai Chi was chosen for use in this study, combined with Thera-band training using the hands. Prior to the pilot RCT study, an online survey was conducted to investigate the physical activity involvement, psychological wellbeing, and pain of older office workers aged 55 years and older, which could provide an insight into the physical activity intervention and workplace health promotion. The online survey reported that 27.3% of the older office workers had low levels of physical activity. Also, depression and anxiety were identified as two common major health issues in older office workers. In the pilot RCT study, 40 office workers aged 55 years and older, who met the inclusion and exclusion criteria, as well as provided signed consent forms, were randomly allocated into either the Tai Chi combined with Thera-band training (intervention) group or the Tai Chi exercise only (control) group. Outcome assessments were conducted at baseline, week 6, and week 12, using the 30-Second Chair Stand Test for lower limb strength, Grip Strength Test for upper limb strength, Function Reach Test for balance, 2-Minute Walk Test for walking endurance, Perceived Stress Scale for stress, Centre for Epidemiological Studies Depression Scale for depression, Geriatric Anxiety Inventory for anxiety, and Visual Analogue Scale for pain. The majority of the participants were female (77.5%), had achieved a higher education qualification (55%), worked full-time (77.5%), were married (62.5%), and recorded as being right-handed (92.5%). A significant difference in pain at baseline was found among participants in the control and intervention groups (p < 0.05). Nine out of the 40 participants withdrew their participation during the study. An overall average attendance rate of 66.7% was recorded across the 40 participants, with 64.3% in the intervention group and 69.1% in the control group. After adjusting for the baseline pain scores and the attendance rate, analysis of data found that there was a statistically significant interaction between intervention type and time with anxiety scores (p = 0.04) and pain scores (p = 0.02). In addition, there were significant differences between groups in the improvement of lower limb strength (p = 0.01) and right upper limb strength (p = 0.04). No significant differences were found in other outcomes; however, the odds ratio estimation indicated that a higher percentage of participants in the intervention group had improvement in balance and walking endurance, and reduction in stress and depression compared with the control group. This study is the first known trial to test the effectiveness of Tai Chi combined with Thera-band training for health promotion in older sedentary office workers when compared to Tai Chi exercise only, with regard to physical fitness, psychological wellbeing, and pain. This study suggested that Tai Chi combined with Thera-band training has the potential to improve lower limb strength and upper limb strength compared to Tai Chi exercise only in older sedentary office workers. Limitations and recommendations from this study may be helpful in designing a large-scale RCT study regarding the effects of the Tai Chi combined with Thera-band training programme on health promotion in older office workers.