Given the difficulties of achieving sufficient load transfer using intermediate modulus carbon fiber-reinforced concrete polymer (IM-CFRP) laminates, limited research has been carried out on strengthening steel structures with CFRP composites. The recent development of a new generation of ultra-high modulus-CFRP (UHM-CFRP) composites has made it possible to broaden the spectrum of potential strengthening applications, particularly those involving structural steel elements. This study investigates the effect of the bond length on the behavior of the adhesively bonded UHM-CFRP to the steel members. In this paper, a total of six adhesively bonded UHM-CFRP/Steel double lab shear (DLS) joint specimens—including three DLS joint specimens with a bond length of 90 mm and three DLS joint specimens with a bond length of 180 mm—were prepared and tested. The experimental results of this study are presented in terms of failure mode, ultimate load carrying capacity, and ultimate longitudinal displacement. The experimental results illustrate that the bond length of 180 mm is the effective bond length for the UHM-CFRP composite strengthening system to the steel members. In addition, the experimental results show that the UHM-CFRP composite system used in this work demonstrates superior performance with a high bond strength and elongation for the UHM-CFRP composite strengthening system to the steel members. The experimental outcomes and conclusions of this work can be implemented in assessing and exploring the feasibility of using UHM-CFRP composites for the strengthening and rehabilitation of steel structures.