Whereas facet joints are recognised as being involved in back pain with high prevalence of early degeneration, the biomechanics of facet joints remain understudied and often overlooked in the evaluation of spinal interventions. This study aimed to develop a methodology for investigating the biomechanics of lumbar facet joints and applied it to a mock fusion model with posteriorly centred compressive loads. The proposed methodology involved measuring facet joint biomechanics through synchronized specimen load and displacement measurements, motion capture of the superior facets with 4 K webcams, and pressure mapping through the facet joints. The experimental method was developed using aged ovine lumbar functional units (N = 6). Results showed that the proposed methodology to measure facet joints biomechanics was accurate (displacement errors below 0.2 mm) and able to capture changes in biomechanics following a mock fusion (with significant differences in all measured displacements). Pressure measurement was challenging due to curvature changes in old ovine tissue which was used for method development but translated successfully to human lumbar tissue. It showed that an aged sheep model is not a good model for posterior spinal biomechanics. This work specifies a new, accurate, methodology to evaluate facet joint biomechanics in vitro and, uniquely, how they change following a spinal intervention.