Many factors influencing second language (L2) speech fluency have been widely studied, but the effects of first language (L1) fluency on L2 speech fluency are still relatively poorly understood. In contrast to mostly quantitative previous studies, the present study adopted a mixed methods approach to examining the connections between L1 fluency and L2 fluency. Monologue speech samples in L1 and L2 were obtained from 42 Finnish learners of English at 2 school levels (9th grade and upper secondary school). The samples were examined for 13 measures capturing different aspects of fluency: temporal fluency, including speed and pausing, and stalling mechanisms. The results indicated positive correlations between the majority of temporal L1 and L2 fluency measures. Regression analyses further demonstrated that most temporal L2 fluency measures could be predicted from L1 fluency measures to a certain extent, although the predictive power varied across the measures. Regarding stalling mechanisms, a complementary qualitative analysis provided insights to idiosyncratic patterns in their use in L1 and L2. Together, the findings suggest that L1 fluency is an important factor in explaining L2 fluency and should be more widely acknowledged in L2 fluency research, assessment, and teaching.