With sustainability becoming a familiar concept in society, higher education institutions have also started playing a more active role in this field, showing an increasing interest in students' comprehension of sustainability. This interest has led to the need for developing methods of assessing students' sustainability knowledge, as observed in many scholars' and institutions' research efforts. To this end, this study aims to evaluate the sustainability literacy levels of students enrolled in a first-year composition course using a mixed-method sequential explanatory design. The quantitative phase involved 221 students who completed a questionnaire assessing their knowledge, skills, attitudes, and familiarity with sustainability concepts and topics. Following this, 60 students were asked to write an essay identifying Kuwait's significant sustainability challenges and proposing solutions. The results showed that gender, high school type, grade point average (GPA) value, and whether they heard about the term sustainability before caused significant differences in students' knowledge, skills, and attitudes about sustainability. Also, the students primarily associate sustainability challenges with environmental problems; social and economic challenges are rarely mentioned in essays.