Recently published experimental data suggest that the Earth’s mantle below ∼250 km is saturated in metallic Fe, reaching a concentration of ∼1 wt.% Fe0 throughout the lower mantle and buffering oxygen fugacity by Fe0↔Fe2+ reactions. Metallic Fe in the mantle bears important implications for N, which behaves as a moderately siderophile element. Here, we propose that the trapping of N in (Fe, Ni) metal and Fe-carbides during diamond growth may account for the characteristically low N content of diamonds from the sublithospheric mantle. This model may also explain the origin of especially valuable, large, anhedral, flawless Type II (N-free) diamonds, like the Cullinan, that comprise a minor part of world diamond production. Partition coefficients of N (DN) between diamond and metallic Fe within DN = 0.0005–0.013 have been demonstrated in high-pressure experiments for diamond synthesis and in a natural sample of a N-poor lower mantle diamond with inclusions of Fe-nitrocarbide. More N is incorporated into diamond if it grows in the lithosphere, where there is no ambient Fe0 to trap N. As a broader implication, the recognition of Fe0 in the mantle and its affinity for N suggests metallic Fe should be a major host of mantle N. Retention of primordial mantle N in metallic Fe could explain the high N/36Ar and low 15N/14N ratios of the mantle compared with the atmosphere.