A facile route for the in situ surface modification and mechanical strength-enhancement of electrospun nanofibers is described. Blends of a host hydrophilic polymer, polysulfone (PSF), and small quantities of a fluorinated polyurethane additive (FPA) up to 6 wt% in the PSF blend were considered. During electrospinning, this additive undergoes spontaneous surface segregation resulting in nanofibers with an improved hydrophobic character attributed mainly to the detected fluorine-rich surface and enhanced mechanical properties. We report the effects of the fluorinated additive content in the blend on the morphological and structural characteristics of the electrospun nanofibrous mats (ENMs) including their inter-fiber space and void volume fraction. ENMs with a superhydrophobic surface could be prepared using at least 3 wt% FPA in the PSF blend. The ENMs were tested in desalination by membrane distillation (MD) and competitive permeate fluxes as high as 53.8 kg/m2h, with stable low permeate electrical conductivities (<5.1 μS/cm), were achieved for the ENM prepared with 4.5 wt% FPA in the PSF blend, when the feed was 30 g/L NaCl aqueous solution and the transmembrane temperature difference was 60 °C, without any inter-fiber space wetting being detected. Display omitted •Surface modification of polysulfone (PSF) nanofiber by surface segregation.•Fluorine enrichment of electrospun PSF nanofiber surface and superhydrophobicity.•Use of small amount of fluorinated polyurethane active additive (FPA).•Suitable characteristics for desalination by direct contact membrane distillation.•Competitive and stable permeate fluxes without interfiber space wetting were achieved.