There are different types of psycholinguistic approaches which attempt to examine the quality and the organization of the human mental lexicon; the word association experiment is one of them. The word association experiment can be used to probe the development of human vocabulary. The current investigation was carried out in order to trace the influence of the cultural background and L2 knowledge on the mental lexicon of the undergraduate Pakistani L2 learners of English. It was hypothesized that the individual‟s culture and knowledge of L2 bear direct relation with their mental lexicon. Influenced by the culture, they may connect different words with attitudinal bonds, whereas L2 knowledge is accountable for the growth of vocabulary. The motivation stems from the fact that none of the previous studies has targeted Pakistani L2 learners for the word association test in order to investigate their mental lexicon. The data was gathered through a word association test. The results supported the hypothesis. A considerable amount of attitudinal responses emerged in their responses, and the number of paradigmatic responses found in the data was the highest of all. Therefore, it was concluded that Pakistani L2 learners‟ vocabulary was considerably influenced by their cultural milieu due to the presence of attitudinal responses to the stimulus words, and their vocabulary is patterning toward native-like since the number of paradigmatic relations with the stimulus words was the highest of other types of relations. The findings carry important implications for didactics.