The mobile environment of the 2020s is experiencing a vertical video revolution. The portrait, or vertical, screen format is replacing the traditional landscape, or horizontal, format to become the default for mobile video production and consumption. With the increasing use of vertical videos, an important, yet unanswered, question is how mobile users respond to this format. Therefore, we examine the effectiveness of mobile vertical versus horizontal video advertisements in terms of consumer interest, engagement, and processing fluency, as well as the underlying mechanism of the effort of watching the video ad on a smartphone in three studies. In a large-scale field study, we demonstrate that mobile vertical video ads increase consumer interest and engagement compared to horizontal video ads. In two experimental studies, we further show that mobile users process vertical video ads more fluently than horizontal video ads. Exploring the underlying mechanism for this effect, we find that mobile users experience less effort when watching a video ad vertically (vs. horizontally) on the smartphone in full-screen, as watching a vertical video does not require turning the phone. Importantly, we find that mobile users' age moderates this indirect effect, as younger mobile users (Generation Z) process mobile vertical video ads more fluently than older Generations X and Y. This article closes with implications for theory and suggestions for mobile marketers.