In the 21st century, the demand for proficiency in mathematics has escalated, as it underpins critical thinking, problem-solving, and innovation in various fields. Recognizing the significance of mathematics education, after-school programs have emerged as potent supplementary platforms to enhance students' mathematical skills and achievement. This study aimed to determine the impact of the after-school program on fostering mathematics in the 21st century. The study was conducted at Chaparral Middle School of Alamogordo Public Schools District in New Mexico, USA to middle school students enrolled for the school year 2023-2024. The research method is experimental. The after-school program students were exposed to the mathematics resources in the 21st century such as Khan Academy, ALEKS, Peer-Assisted Learning Strategies, Illustrative Math, and the like. Evidence-based resources were used to foster students' mathematics achievement. A standardized test that measures mathematics achievement was the main instrument used in the study. The mean, standard deviation, and t-test were used to analyze the data collected. The analysis with a t-value of 7.889 and probability value of 0.000 revealed that students exposed to the after-school program performed significantly in terms of mathematics achievement. Researchers recommend that teachers may use the after-school program to foster mathematics achievement. Teachers might also incorporate activities into their Mathematics teaching that demand students to engage resource materials to foster students' mathematics potential in the 21st century.