Background: We aim to evaluate the influence of the timing of leptomeningeal metastasis (LM) occurrence on the outcome of EGFR-mutant lung adenocarcinoma and to explore the predictors of detectable EGFR mutation in the cerebrospinal fluid (CSF). Methods: EGFR-mutant lung adenocarcinoma patients with cytologically confirmed LM were included for analysis. EGFR mutation in CSF was detected by MALDI-TOF MS plus PNA. Results: A total of 43 patients was analyzed. Of them, 8 (18.6%) were diagnosed with LM prior to first-line EGFR-TKI treatment (early onset), while 35 patients (81.4%) developed LM after first-line EGFR-TKI treatment (late onset). Multivariate analysis suggested that both late-onset LM (aHR 0.31 (95% CI 0.10–0.94), p = 0.038) and a history of third-generation EGFR-TKI treatment (aHR 0.24 (95% CI 0.09–0.67), p = 0.006) independently predicted a favorable outcome. EGFR mutation detection sensitivity in CSF was 81.4%. The radiological burden of LM significantly correlated with CSF tumor cell counts (p = 0.013) with higher CSF tumor cell counts predicting a higher detection sensitivity of EGFR mutation (p = 0.042). Conclusions: Early onset LM was an independently poor prognostic factor. A higher radiological severity score of LM could predict higher tumor cell counts in CSF, which in turn were associated with a higher detection rate of EGFR mutation.