Memory dysfunction in mild cognitive impairment (MCI) due to Alzheimer's pathology is primarily associated with episodic memory deficits linked to deterioration of the medial temporal lobes (MTLs). Currently, there is a call to discover novel biomarkers of MCI in order to improve research criteria. Functional activation differences in MCI during episodic memory-task performance are often evidenced in the MTLs, and frontal and parietal lobes, but it has been suggested that examination of working memory (WM) differences may be more useful in detecting MCI. In the current study, MCI and control participants performed a complex WM span (CWMS) task while functional magnetic resonance imaging (fMRI) data were acquired. Results indicated hyper-activation of the lateral temporal lobes, MTLs, and frontal and parietal regions during encoding and maintenance, and hyper-activation of the lateral temporal, frontal, and parietal lobes during CWMS recall for the MCI participants. Medial and lateral temporal differences during encoding and maintenance are consistent with previous findings, but lateral temporal differences are often not elaborated upon. Hyper-activation of the lateral temporal lobes during WM encoding and maintenance, and also during recall, suggests that this region may provide valuable information regarding WM impairment in MCI and Alzheimer's. Given that whole-brain functional imaging of the MTLs is often limited due to artifact and partial voluming of sub-fields, examination of lateral temporal differences may provide a novel biomarker related to WM impairment in MCI.