The out-of-plane behaviour of unreinforced masonry (URM) walls strengthened using near surface mounting of twisted steel bars (NSM-TS) was investigated by performing two series of tests, which involved in situ and laboratory based out-of-plane testing of full scale strengthened URM walls. In the first series of testing, two walls were strengthened and tested in situ inside an historic URM house located in Wellington (New Zealand), known to be originally constructed in 1884 and to have undergone several minor periodic alterations. In the second series of testing, the results of in situ testing were further confirmed by performing laboratory based reverse cyclic out-of-plane testing of two slender URM walls that were constructed using vintage solid clay bricks and a low strength hydraulic mortar, replicating typical historic URM construction. Numerous parameters pertaining to the out-of-plane behaviour of NSM-TS strengthened URM walls were investigated, including failure modes, hysteretic response curves, out-of-plane flexural strength, maximum drift, pseudo-ductility, and strain distribution in the NSM-TS bars. Finally, measured performance parameters from the strengthened walls were compared to the corresponding data from the as-built tested walls. It was inferred from the results that the observed flexural strength increase due to NSM-TS strengthening ranged from 143% to 434% when compared to the strength of as-built URM wall.