With benefits to both human well-being and pro-nature conservation behaviors, nature connectedness is emerging as an important psychological construct for a sustainable future. The growing research and applied and policy-related interests require a straightforward measure of nature connectedness that is suitable for both children and adult populations. To establish the reliability of the new Nature Connection Index (NCI) three factor analyses were conducted. One was based on a large Monitor of Engagement with the Natural Environment (MENE) dataset for adults (n = 3568) with a replication from data sets collected online (n = 553), and a third used MENE data from children (n = 351). To validate the NCI as a measure for nature connectedness an online comparison study (n = 153) included the NCI alongside other established measures. The results showed that the NCI was a reliable and valid scale that offers a short, simple alternative to other measures of nature connectedness, particularly for populations including both children and adults, measured face to face or online. The utility of the NCI is also supported, with variations associated with various pro-environmental and pro-conservation behaviors observed, and importantly the NCI also revealed changes in nature connectedness across the lifespan.