For restoring historical timber structures under the principles of minimal intervention, reversibility, and recognizability, externally bonded (EB) steel plate and near-surface mounted (NSM) CFRP bars are often adopted when strengthening timber beams. To investigate the long-term behaviour of timber beams strengthened with externally bonded (EB) steel plate and near-surface mounted (NSM) CFRP bars, three full-size timber beam specimens were designed and subjected to sustained loading for 3000 days. The test results indicated that the initial defects in the tension zone at mid-span of strengthened specimen played an important role in long-term behaviour of timber beams. The deflection of strengthened beam without defects at mid-span was less than that of the unstrengthened control beam as expected. The deflection of strengthened beam with defects at mid-span was much higher than that of the unstrengthened control beam, with visible cracks occurred around the knot at mid-span of the strengthened beam during sustained loading. In addition, the primary creep stage and stable creep stage were clearly observed in all the specimens, and accelerated creep stage was found in the specimen with initial knots in the tension zone at mid-span. The wood strain was observed to generally increase as temperature and moisture content increased. The material parameters of classical creep models for the control beam and timber beams strengthened with different methods were determined based on fitting with the test results. Finite element models of timber beams strengthened with EB steel plate and NSM CFRP bars were developed, and then the validated model was used for parametric study. The finite element model of strengthened timber beam without defects at mid-span exhibited fair agreement with the test results. Parametric analysis implied that the thickness of steel plate, the diameter and number of CFRP bars played a significant role in the deflection of the strengthened timber beams under sustained loading.