A major change in Earth's geodynamics occurred ~3billionyears (Ga) ago, likely related to the onset of modern and continuous plate tectonics. However, the question of how Earth functioned prior to this time is poorly constrained. Here, we find a resolvable positive 142Nd anomaly in a 2.7Ga old tholeiitic lava flow from the Abitibi Greenstone Belt indicating that early-formed mantle heterogeneities persisted at least 1.8Ga after Earth's formation. This result contradicts the expected rapid early (~0.1Ga), as well as the slower present-day (~1Ga) mixing rates in the convecting mantle. Using a numerical modeling approach, we show that convective mixing is inefficient in absence of mobile-lid plate tectonics. The preservation of a 142Nd anomaly until 2.7Ga ago can be explained if throughout the Hadean and Archean, Earth was characterized by a stagnant-lid regime, possibly with sporadic and short subduction episodes. The major change in geodynamics observed around ~3Ga ago can then reflect the transition from stagnant-lid plate tectonics to modern mobile-lid plate tectonics. Solving the paradox of a convective but poorly-mixed mantle has implications not only for Archean Earth, but also for other planets in the solar system such as Mars.