In the present randomized controlled trial (RCT) it was investigated whether single women, 55 years of age and older, improved with regard to self-management ability, well-being, and social and emotional loneliness after having participated in a newly designed self-management group intervention based on the Self-Management of Well-being (SMW) theory. The expected mediating effect of self-management ability on well-being was not found. Although self-management ability, well-being and loneliness improved significantly in the intervention group immediately after the intervention, and also remained at this improved level after six months, there was also improvement in the control group after six months, rendering the longer-term differences between the groups non-significant. It can, however, be concluded that, although the longer-term effectiveness could not be proven, this SMW theory-based intervention seems to be useful in supporting older women to improve their self-management ability and well-being.