Steel structures frequently experience extreme service conditions such as fire accidents, corrosion, etc., resulting in significant deterioration, reduced service life, and increased maintenance cost. Almost 0.5 million structural fire accidents are reported annually in the U.S., inflicting a considerable toll on infrastructure. Similarly, corrosion-induced deterioration is the leading cause of premature failure in infrastructure and costs $22.6 billion in infrastructure maintenance annually. The bridge infrastructure vulnerability to these extreme service conditions is compounded by the aging bridge inventory (42% of the 617,000 bridges in the U.S. are 50 years or older). To improve the resilience and the “C-” infrastructure grade, it is vital to understand the material-scale damages and mechanisms induced by such extreme service conditions and develop efficient mitigation strategies. This dissertation adopts a two-prong approach to understand and predict the material-scale damage after exposure to fire accidents in steel structures and mitigate corrosion damage in steel/RC structures. Specifically, it aims to improve our current understanding of the post-fire mechanical behavior of steels and propose a microstructure-based approach for forensic analysis of fire-affected steel structures in phase-I. It further aims to mitigate corrosion in steel/RC structures by employing agriculturally-derived non-toxic materials and surface treatments in phase-II. Post-fire mechanical and microstructural investigations conducted in phase-I revealed that stress concentrations and fire-extinguishing methods significantly affect the post-fire mechanical behavior of structural steels, and post-fire steel microstructure can be utilized to accurately estimate the mechanical strength of structural steels without the knowledge of fire temperatures. The outcomes of phase-I of this dissertation can lead to accurate forensic fire investigations and usability determination of the fire-affected steel structures. The results obtained from phase-II of this dissertation revealed the role of surface treatments in improving the corrosion resistance and validated the performance of the agriculturally-derived materials such as corn-derived inhibitors and soy-protein coatings in lowering the corrosion damage in structural steels and embedded rebars by up to 90% without compromising the integrity of cement-based materials. These outcomes will lead to mitigating corrosion-induced deterioration in aging and new infrastructure. Overall, the outcomes of this dissertation contribute to improving infrastructure resilience and reducing maintenance costs.