The evolvement of smart cementitious materials with self-monitoring features, evaluating and early warning the structural integrity of infrastructures at an early stage is of great importance both for the service and sustainability aspects. This feasibility study investigates two grades of commercially available continuous single carbon nanotube fiber (CNTF) filaments embedded in a sustainable geopolymer (GP) matrix to assess their potential as piezoresistive elements for mechanical deformation sensing via straightforward two-probe DC electrical resistance measurements. Prior their incorporation, the CNTF filaments were systematically characterized. Afterwards, smart composites were prepared by a simple casting method around the CNTF and the GP reaction was monitored. Thereby, a prominent change of the pure electronic electrical properties was identified upon ageing, exhibiting similar electrical resistance evolution profiles, capturing the transition from fresh mixture to a rigid matrix within 24 h. Subsequently, the integrated CNTFs were utilized as mechanical stress sensors during static and cyclic three-point-bending, as well as compression loading conditions, respectively. The obtained results demonstrate the ability to reliably detect applied mechanical loads through the electrical signal, expressed as the fractional change in electrical resistance (FCR) combined with efficient electrical current transmission independently of environmental influencing factors. Importantly, cyclic compression tests confirmed highly reversible and repeatable electrical responses achieving circa 10% FCR. The outcome indicates that such CNTF/GP multifunctional building materials are capable to be used as durable, built-in and real-time structural health monitoring (SHM) indicators to enable an autonomous maintenance of civil infrastructures.