Abstract The Kuiper Belt objects (KBOs), the Centaurs, and the Jupiter-family comets (JFCs) form an evolutionary continuum of small outer solar system objects, and their study allows us to gain insight into the history and evolution of the solar system. Broadband photometry can be used to measure their phase curves, allowing a first-order probe into the surface properties of these objects, though limited telescope time makes measuring accurate phase curves difficult. We make use of serendipitous broadband photometry from the long-baseline, high-cadence Asteroid Terrestrial-impact Last Alert System survey to measure the phase curves for a sample of 18 KBOs, Centaurs, and JFCs with unprecedentedly large data sets. We find phase curves with previously reported negative slopes become positive with increased data and are thus due to insufficient sampling of the phase-curve profile, and not a real physical effect. We search for correlations between phase-curve parameters, finding no strong correlations between any parameter pair, consistent with the findings of previous studies. We search for instances of cometary activity in our sample, finding a previously reported outburst by Echeclus and a new epoch of increased activity by Chiron. Applying the main belt asteroid HG ₁ G ₂ phase-curve model to three JFCs in our sample with large phase angle spans, we find their slope parameters imply surfaces more consistent with those of carbonaceous main belt asteroids than silicaceous ones.