The health and wellbeing benefits of engaging in physical activity (PA), and of improving strength and balance, are well documented. The World Health Organization's recommendations of 150 min per week of moderate intensity physical activity have been adopted across the world in policy and practice recommendations. However, the number of older adults engaging in this level of PA remains low. The European Project, PreventIT, has adapted the Lifestyle-integrated Functional Exercise (LiFE) programme, which reduced falls in people 75 years and over, for a younger cohort (aLiFE). aLiFE incorporates challenging strength and balance/agility tasks, as well as specific recommendations for increasing physical activity and reducing sedentary behaviour in young-older adults, aged 60–70 years. Personalised advice is given on how to integrate strength, balance and physical activities into daily life. aLiFE has been further developed to be delivered using smartphones and smartwatches (eLiFE), providing the opportunity to send timely motivational messages and real-time feedback to the user. Both aLiFE and eLiFE are behaviour change interventions, supporting older adults to form long-term physical activity habits. PreventIT has taken the original LiFE concept and further developed the behaviour change elements, explicitly mapping them to Social Cognitive Theory, Habit Formation Theory and 30 Behaviour Change Techniques (BCTs). Goal setting, planning, prompts and real-time feedback are used to deliver a person-centred experience. Over 1300 motivational messages have been written, mapped to psychological theory, BCTs and evidence regarding the importance of strength, balance and PA. A motivational assessment tool has been developed to enable us to investigate stated motivational drivers with actual performed behaviour within the feasibility Randomised Controlled Trial. The PreventIT mHealth intervention focusses on behaviour change from initiation to long-term maintenance, addressing the different phases of adopting a healthier lifestyle. As such, it makes a strong contribution to the developing field of evidence-based mobile health (mHealth).