For a minority of the bereaved, the loss of a significant other can trigger an overwhelming emotional reaction and impaired functioning across life domains, known as prolonged grief disorder (PGD). Hence, ongoing efforts have been made to refine existing treatments to increase their efficacy and to accommodate the idiosyncrasies of grief reactions. This study presents the results of an open clinical trial of the feasibility and effectiveness of the Meaning in Loss (MIL) protocol in an online format. The brief intervention of 12 to 16 sessions combines constructivist and narrative strategies to explore and work through impediments to meaning reconstruction in loss. The sample included 25 participants diagnosed with PGD who were treated by six therapists. Baseline and post-therapy comparisons showed a significant improvement in all clinical measures (grief symptomatology, depression and general distress) and an increase of meaning making regarding the loss. Meaning making was found to be a prospective mediator of symptomatic improvement in grief across the course of therapy. These findings suggest the effectiveness of the MIL protocol in decreasing grief specific and associated symptomatology and argue for the relevance of further controlled evaluations of its efficacy. Moreover, results confirm previous findings that meaning making is a relevant factor in the evolution of grief reactions, including in the context of psychotherapy.