We have investigated the effect of an assembly history on the evolution of galactic dark matter (DM) halos of 10¹² h^-1 M using constrained realizations of random Gaussian fields. Five different realizations of a DM halo with distinct merging histories were constructed and have been evolved using collisionless high-resolution N-body simulations. Our main results are as follows: A halo evolves via a sequence of quiescent phases of a slow mass accretion intermitted by violent episodes of major mergers. In the quiescent phases, the density is well fitted by an NFW profile, the inner scale radius R_s and the mass enclosed within it remain constant, and the virial radius (R_vir) grows linearly with the expansion parameter a. Within each quiescent phase the concentration parameter (c) scales as a, and the mass accretion history (M_vir) is well described by the Tasitsiomi et al. fitting formula. In the violent phases the halos are not in a virial dynamical equilibrium and both R_s and R_vir grow discontinuously. The violent episodes drive the halos from one NFW dynamical equilibrium to another. The final structure of a halo, including c, depends on the degree of violence of the major mergers and the number of violent events. Next, we find a distinct difference between the behavior of various NFW parameters taken as averages over an ensemble of halos and those of individual halos. Moreover, the simple scaling relations c-M_vir do not apply to the entire evolution of individual halos, and therefore we have the common notion that late-forming halos are less concentrated than early-forming ones. The entire evolution of the halo cannot be fitted by single analytical expressions.