Nanodiamonds containing negatively charged nitrogen vacancy centers (${\text{NV}}^{\ensuremath{-}}$) have applications as localized sensors in biological materials and have been proposed as a platform to probe the macroscopic limits of spatial superposition and the quantum nature of gravity. A key requirement for these applications is to obtain nanodiamonds containing ${\text{NV}}^{\ensuremath{-}}$ with long spin coherence times. Using milling to fabricate nanodiamonds processes the full 3D volume of the bulk material at once, unlike etching pillars, but has, up to now, limited ${\text{NV}}^{\ensuremath{-}}$ spin coherence times. Here, we use natural isotopic abundance nanodiamonds produced by ${\text{Si}}_{3}{\text{N}}_{4}$ ball milling of chemical vapor deposition grown bulk diamond with an average single substitutional nitrogen concentration of $121\phantom{\rule{0.16em}{0ex}}\text{ppb}$. We show that the electron spin coherence times of ${\text{NV}}^{\ensuremath{-}}$ centers in these nanodiamonds can exceed 400 $\ensuremath{\mu}\text{s}$ at room temperature with dynamical decoupling. Scanning electron microscopy provides images of the specific nanodiamonds containing ${\text{NV}}^{\ensuremath{-}}$ for which a spin coherence time was measured.