A facile methodology to durably modify cellulose fibers by forming in-situ phosphine oxide macromolecular physical networks is developed in this work. Such networks were formed by treating the cellulose fibers with a mixture of Michael adducts [trivinyl phosphine oxide (TVPO) and cyclic amines (piperazine and 2,4,6-tri(piperazine-1-yl)-1,3,5-triazine)] followed by a click crosslinking reaction initiated by dry heat, microwave, or steaming process. The in-situ phosphine oxide network is integrated within the cellulose fibers to produce a durable treatment against laundering. NMR and SEM analysis of the treated fibers verified the in-situ formation of phosphine oxide macromolecules in the bulk of cellulose fiber. The treatment was homogenous and achieved an outstanding phosphorus (P) retention (>95%) after even 50 laundry cycles. The treated cotton exhibited excellent fire retardant properties with a limiting oxygen index value > 27 % and passed the Swiss vertical flammability test at 2 wt.% P. Analysis of evolved gasses during thermal decomposition of treated cellulose supported both the condensed and gas phases mechanism. The flame retardant treatment was further adapted to include antimicrobial properties by in situ silver incorporation of silver nanoparticles within the cellulose fibers using a single-step process to demonstrate this novel fiber treatment methodology's versatility. The treated fabric exhibited excellent antimicrobial and flame retardant properties and was also durable to 50 laundry cycles. Contact angle measurements and moisture management test confirmed hydrophilicity and the treated cellulose's excellent comfort behavior.