In 1997 the US established the Child Tax Credit (CTC), which offers payments to parents of dependent children to help defray child-rearing costs. In 2021 a temporary expansion to the CTC increased the size of payments, extended payments to families with low or no earnings, and distributed payments monthly instead of annually. Quasi-experimental evidence from the US and experimental evidence from low- and middle-income countries shows that moderate-to-large cash transfers improve subjective well-being and mental health. We estimated the CTC's expansion's effects on the subjective well-being and mental health of adult recipients, using data from the Understanding America Study, a nationally representative survey with more than 7,000 respondents and more than 2,700 unique respondents with children. We found no evidence that the CTC expansion had a significant short-term impact on measures of life satisfaction, anxiety, and depression symptomology among adult recipients. We speculate that the null effects may be due to the expansion's temporary nature.