Bipolar disorder is a serious and impairing mental illness that affects up to 4% of the US population. Early onset (before age 21) suggests a more genetically driven form of the disorder and is associated with a poorer prognosis. Both genetic and environmental effects are recognized as significant contributors to the etiology of this disorder. The goal of this study was to identify gene-environment interactions (GxE) that increase or decrease the likelihood of developing early onset bipolar disorder symptomatology in late adolescence. Potential mechanisms behind the hypothesized GxE were also tested. Participants provided DNA for the sequencing of the serotonin transporter promoter polymorphism (HTTLPR), filled out questionnaires regarding retrospective family functioning and their current mood state, and completed an affective memory task measuring biased attention towards emotional stimuli. Family environment factors such as positive expressed emotion, family closeness, and conflict predicted changes in depression and/or mania symptoms. Moderated mediation models involving attention to emotion faces were not significant. There were no statistically significant gene-environment interactions, most likely due to the study being underpowered. Results from this study contribute to a growing body of knowledge regarding best practices and feasibility around gene-environment research.