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Most of the large rocky bodies in the solar system display evidence of past and/or current magnetic activity, driven by thermochemical convection in an electrically conducting fluid layer. The discovery of a large number of extrasolar planets motivates the search for magnetic fields beyond the solar system. While current observations are limited to providing planetary radii and minimum masses, studying the evolution of exoplanets' magnetic fields and their interaction with the atmosphere can open new avenues for constraining interior properties from future atmospheric observations. Here, we investigate the evolution of massive rocky planets (0.8 − 2 MEarth) with different bulk and mantle iron contents. Starting from their temperature profiles after accretion, we determine the structure of the core and model its subsequent thermal and magnetic evolution over 5 Gyr. We find that the planetary iron inventory and distribution strongly affect core structure, evolution, and the lifetime of a magnetic field. Planets with large bulk and mantle iron contents tend to feature large solid inner cores, which can grow up to the liquid outer core radius, shutting down any pre‐existing magnetic activity. Consequently, the longest dynamo lifetimes (∼ 4.25 Gyr) are obtained for massive planets with intermediate iron inventories. The smaller inner core radii and the chemical buoyancy fluxes introduced by the presence of light impurities can extend the magnetic field lifetimes to more than 5 Gyr. While the calculated magnetic fields are too weak to be detected by ground facilities, indirect observations may provide valuable insights into exoplanetary dynamos. Plain Language Summary: Earth's magnetic field is powered by vigorous convection in its liquid metallic outer core. The presence of a magnetic field is thought to accommodate habitable surface conditions by shielding the planetary upper atmosphere from harmful solar radiation. Most rocky planets in our solar system display past or present signatures of magnetic activity, and similar trends may exist in exoplanetary systems. The current knowledge of exoplanets relies on their radii and masses, while internal properties remain largely unconstrained. Studying the evolution of exoplanetary magnetic fields and their interaction with the surrounding environment will help to constrain interior properties from future atmospheric observations. Here, we investigate the structure and the thermal and magnetic evolution of the cores of rocky planets with different masses (0.8–2 Earth masses) and variable iron contents. We find that the iron content and its internal distribution between a planet's core and mantle strongly affect the evolution of the core and the lifetime of a magnetic field. The longest‐lived magnetic fields are obtained for massive planets having intermediate iron contents. Iron‐rich planets tend to grow fully solid cores, hindering any further magnetic activity. The presence of a small fraction of light core impurities can help prolong magnetic field lifetimes. Key Points: We investigate the evolution of the cores of rocky planets with masses 0.8–2 MEarth and variable bulk and mantle iron contentsThe content and distribution of iron in a planetary body significantly influence core evolution and magnetic field lifetimesThe cores of iron‐rich planets tend to become fully solid, shutting off any pre‐existing magnetic field and shortening the dynamo lifetime [ABSTRACT FROM AUTHOR]