Follicular lymphoma (FL) is the most common low-grade lymphoma. Despite its indolent nature, FL carries an inherent risk of histological transformation (HT) to a more aggressive lymphoma. Existing biomarkers are insufficient to predict HT, indicating the need for more robust biological predictors. Previously, we used mass spectrometry-based proteomics to identify differentially expressed proteins in diagnostic FLs with and without subsequent HT. This study sought to further investigate identified proteins in transformation of FL, generally acting in important cellular pathways such as (i) apoptosis (BID), (ii) cell cycle (CDC26, CDK6, SRSF1, SRSF2), (iii) GTPase signaling (IQGAP2, MEK1), (iv) cytoskeletal rearrangement and cellular migration (ACTB, CD11a, MMP9, SEPT6), and (v) immune processes (CD81, IgG, MPO, PIK3AP1). We analyzed pre-therapeutic samples from 48 FL patients, either non-transforming FL (nt-FL, n = 30) or subsequently-transforming FL (st-FL, n = 18), the latter with histologically-confirmed transformation after their initial FL diagnosis. Paired high-grade lymphomas (tFL, n = 18) from the time of transformation were also analyzed. We used immunohistochemistry and digital image analysis to quantify protein levels. In all five pathway classes, several proteins were differentially expressed between either the diagnostic nt-FL and st-FL samples, or between the paired st-FL and tFL samples (p < 0.05). Interestingly, we found correlation between expression levels of several proteins, indicating a complex involvement between several pathways. Differential expression of most proteins was also associated with shorter transformation-free survival (p < 0.05). These findings emphasize underlying differences in FL biology predictive of subsequent transformation, highlighting deregulation of important interconnected cellular pathways.