Fines concentration harms paper machine runability and output quality in recovered paper processing, hence, their extraction would be fundamentally beneficial. In this study, separated fines from an industrial recycled fiber pulp (RFP) were characterized and evaluated for their potential biogas yields with a focus on understanding the role of varying lignin and ash contents. Further, these results were compared with biogas yields from conventional chemical and mechanical pulps. Overall, methane yields of fines from mechanical pulps (21–28 mL/gVS) and RFP (127 mL/gVS) are relatively low compared to the high methane yields of 375 mL/gVS from the chemical pulp fines. However, it was shown that the high ash content in RFP fines (up to 50%) did not negatively influence overall yield, rather, it was the presence of slowly biodegrading lignin-rich fiber fines.