Digital Storytelling (DS) is an alternative instructional tool for language teaching to make the relevant process more engaging through the use of technology and to facilitate effective language learning. This single case study explored the impact of DS in an online EFL lesson on speaking and vocabulary achievement in consideration to cognitive load effect. The study utilized mixed-design analysis strategies as quantitative tests (pre- and post-vocabulary test, speaking evaluation rubric, cognitive load scale) with 33 Turkish EFL learners studying at tertiary level who participated in an online English course and interviews with 21 volunteers. Quantitative findings revealed increased vocabulary achievement after DS use and improved English-speaking skills in terms of spoken grammar, vocabulary use, fluency and coherence, and pronunciation after repeated DS use in three stories, despite high level of cognitive load due to technical incompetency of the participants. In addition, the results obtained from the qualitative data match with the results of the quantitative data in terms of positive evaluations on the impact of DS for vocabulary use, error correction, sentence formation, pronunciation, and self-study. DS can also be effectively used in online EFL education by eliminating the drawbacks of students' digital incompetency at tertiary level. The results of this study are expected to shed light on the practitioners aiming to use DS in online language learning for teaching speaking and improving vocabulary achievement.