The hydrophobic barrier of the membrane prevents simple cellular uptake of many useful exogenous polar compounds in the laboratory and clinic. For the past few decades, cell-penetrating peptides (CPPs) have been intensively studied as a potential cargo delivery vehicle. However, it has gradually been accepted that the actions of CCPs rely on one or multiple types of endocytic pathways, which can make cargo delivery quite inefficient under some conditions. Therefore, this work presented here was initiated to engineer spontaneous membrane-translocating peptides (SMTPs) that move across lipid bilayers and cellular membranes in an endocytosis-independent manner. Previously we used high throughput, orthogonal screen of a synthetic peptide library and identified a family of 12-residue SMTPs that translocate rapidly without causing any bilayer destabilization. Here we conjugated one of the SMTPs described above with several membrane-impermeant molecules and measured the rate of translocation in large unilamellar vesicles. Furthermore, we characterized the cytosolic entry of SMTP-polar cargo conjugates as well as their toxicity in living mammalian cells. We observed rapid delivery of these conjugates with higher efficiency compared to control CPP-cargo conjugates, and little or no evidence for the involvement of endocytosis in SMTP translocation. Meanwhile, we found no measureable cytolytic activity and extremely low cytotoxicity of SMTPs. Upon injection into mice, SMTPs with their cargos were found in many tissues after 2 hours, while free cargo molecules were rapidly cleared and not systemically distributed. Our results show that SMTPs behave in a unique way compared to conventional CPPs: they may strategically elude the dependence of endocytosis in cellular translocation and significantly improve cytosolic delivery of polar cargos.