To become fluent in a second language, learners need to acquire a large vocabulary. However, the cognitive and affective mechanisms that support word learning, particularly among second language learners, are only beginning to be understood. Prior research has focused on intentional learning and small artificial lexicons. In the current study investigating the sources of individual variability in word learning and their underlying mechanisms, participants intentionally and incidentally learned a large vocabulary of Welsh words (i.e., emulating word learning in the wild) and completed a large battery of cognitive and affective measures. The results showed that, for both learning conditions, native language knowledge, auditory/phonological abilities and orthographic sensitivity all made unique contributions to word learning. Importantly, short-term/working memory played a significantly larger role in intentional learning. We discuss these results in the context of the mechanisms that support both native and non-native language learning.