Background: Co-occurring mutations in KRAS-mutant NSCLC are associated with discrete biological properties and modulate therapeutic susceptibilities. As G12D-specific inhibitors are expected to enter the clinic, we sought to define the clinicopathologic characteristics and outcomes of patients (pts) with KRAS G12D-mutant NSCLC. Methods: This was a retrospective single-institution study. Pts with NSCLC and KRAS G12D mutations detected by the Massachusetts General Hospital SNaPshot next-generation sequencing assay were identified. Clinical and pathologic characteristics, including co-mutation status and survival outcomes, were collected by chart review. Results: Among pts who underwent SNaPshot testing between May 2014 and August 2021, 665 had cancers that were positive for KRAS G12D, of whom 107 (16.1%) had NSCLC. Most common histology was adenocarcinoma (93, 86.9%). PD-L1 level was < 1% in 24 (22.4%), 1-49% in 19 (17.8%), > 50% in 13 (12.2%), and not assessed in 51 (47.7%). Co-occurring mutations were frequent: 17 NSCLCs had co-occurring STK11 mutations (15.9%), 36 (33.6%) TP53 mutations, and 11 (10.3%) SMARCA4 mutations, of which 2 were truncating. Genetic alterations were detected in a variety of other genes including in the RTK/RAS/MAPK (20, 18.7%), PI3K/AKT/mTOR (12, 11.2%), cell-cycle (17, 15.9%), and WNT (11, 10.3%) pathways. Among 58 tumors with KEAP1 mutation testing, 10 (17.2%) were positive. 75 (70.1%) pts had metastatic disease, including 51 pts who had metastatic disease at initial diagnosis. CNS metastases were present in 27 (36.0%) pts, including 14 pts with CNS metastases at initial diagnosis. Among the 57 pts with metastatic disease who received first-line therapy, 28 (49.1%) received platinum-based chemotherapy, 17 (29.8%) immunotherapy alone, and 12 (21.1%) platinum chemotherapy plus immunotherapy. Median PFS in pts treated with first-line therapy was 4.0 months (95% CI 2.1– 5.7) and median OS 14.3 months (95% CI 8.1 – 27.4). When stratified by co-mutation status, pts with STK11 mutations had significantly worse outcomes than STK11-wild-type (median PFS: 1.2 months vs 4.8 months, p = 0.0165; median OS: 4.3 months vs 21.6 months, p = 0.0015), whereas TP53 exerted no influence. Conclusions: Co-occurring mutations were common in pts with KRAS G12D-mutant NSCLC. Outcomes were poorer for KRAS G12D/STK11 pts, whereas co-occurring TP53 did not impact survival. Clinical trials examining G12D inhibitors should stratify by STK11.