With the worldwide transformation to a circular and low-carbon economy, the demand for sustainable materials has skyrocketed in recent years. Of various methods, sustainable and biodegradable biopolymers derived from renewable bioresources have received significant interest. Synthetic biodegradable biopolymers offer tremendous advantages over natural biodegradable biopolymers due to their stability, flexibility, and a wide range of achievable properties to fit several applications. However, the widespread adoption of synthetic biodegradable polymers in high-performance applications is limited by shortcomings in their functional properties. Researchers are actively working to enhance the properties of these materials. A potential solution to improve the performance of biopolymers is to reinforce them with cellulose nanocrystals (CNCs). This review delves into the inclusion of CNCs into synthetic biodegradable biopolymer blends, examining their impact on the mechanical, thermal, morphological, rheological, and barrier properties. Surface modification of CNCs promotes a uniform distribution and strong bonding with the polymer matrix which is pivotal to unlocking their outstanding properties. Moreover, this review highlights the promising potential of CNCs to enhance the performance of synthetic biodegradable composites for a more sustainable future, particularly in packaging applications.