Mobile Ad hoc Networks (MANETs) have been very popular for some years now owing to their ability to allow communication in dynamic and infrastructure-less environments. However, the unique characteristics of MANETs, such as node mobility, limited power resources, and absence of centralized infrastructure, pose challenges in ensuring efficient routing and robust security. This paper presents a review of the existing techniques aimed at improving routing protocols and security in MANETs. Scholarly articles, conference papers, and technical reports published in notable research outlets were sourced. Then, the papers were categorized into two main areas: routing techniques and security mechanisms based on the target of this work. Regarding routing techniques, this review discusses the evolution of traditional routing protocols, including proactive, reactive, and hybrid approaches, and highlights their strengths and limitations. Moreover, the review presents some recent advancement such as location-based, Quality of Service (QoS)-aware, and energy-efficient routing protocols, which address specific challenges in MANETs. In terms of security mechanisms, this review provides an overview of the various threats and attacks that MANETs are susceptible to, including black hole, wormhole, and Sybil attacks. The review then examines the countermeasures proposed in the literature to tackle the security challenges. This article further highlights the emerging trends and research directions in the field of MANETs which include blockchain-based security, machine learning-assisted routing, and Internet of Things (IoT) integration. It is believed that this review can provide further insights to researchers in this domain.