As COVID-19 converges with loneliness and addiction epidemics in the US, both public health and mental health experts forecast dramatic increases in substance use and mental health conditions. This cross-sectional study evaluated relationships of loneliness with depression, anxiety, alcohol use, and drug use during COVID-19, and assessed perceived increases in these symptoms in young adults. Between April 22 and May 11, 2020, 1,008 participants ages 18-35 were recruited through social media to a one-time, online anonymous survey. Symptomatology was assessed using six scales. Perceived changes since COVID-19 were evaluated using 5-point Likert scales. Forty-nine percent of respondents reported loneliness scores above 50; 80% reported significant depressive symptoms; 61% reported moderate to severe anxiety; 30% disclosed harmful levels of drinking. While only 22% of the population reported using drugs, 38% reported severe drug use. Loneliness was associated with higher levels of mental health symptomatology. Participants reported significant increases across mental health and substance use symptoms since COVID-19. While direct impacts of COVID-19 could only be calculated with pre-pandemic assessments of these symptoms, estimates indicate elevated psychosocial symptomatology and suggest that symptoms could have worsened since the pandemic. Findings underscore the importance of prevention and intervention to address these public health problems.